The filtering and sorting seem to work fine, however on initial page load, and when attempting to filter/sort through the items only half of the posts are being displayed, 70, out of 140.
The values such as _discount_price or percentage are available in the DB for all the posts, on some of the posts product_categories are not assigned, however I don’t think that this is causing the posts not to load.

I’ve been at this problem for couple of days and I cannot pinpoint where I am making a mistake. Any suggestion/assistance would be welcomed.

This is the form

<form action="<?php echo home_url('/'); ?>" method="get" class="searchandfilter">
            <div class="search-form">
                <input type="text" name="s" placeholder="Search..." class="search-input">

            </div>
            <button class="filterButton" type="button"><span>Filter & Sort</span></button>

            <div id="discount-filter" class="hidden">
                <h2 class="form-title">Percentage Discount</h2>
                <input type="range" id="discountRange" name="discountRange" min="0" max="100" value="20" class="slider">
                <span id="discountValue" aria-live="assertive">20%</span>
            </div>
        
            <div id="status-filter" class="hidden">
                <h2 class="form-title">Status</h2>
                <div class="input-group">
                    <input type="checkbox" name="status[]" value="active-deals" id="statusActive" class="styled-checkbox"><label for="statusActive">Active</label>
                </div>
                <div class="input-group">
                    <input type="checkbox" name="status[]" value="archived-deals" id="statusArchived" class="styled-checkbox"><label for="statusArchived">Archived</label>

                </div>
            </div>
        
            <div id="categories-filter" class="hidden">
                <h2 class="form-title">Categories</h2>
                <div class="category-list">
                <?= display_categories_recursive(0); ?> 
                </div>
                <button id="see-more-categories" type="button" class="see-more" data-expanded="false">See More...</button>
            </div>
        
            <div id="stores-filter" class="hidden">
                <h2 class="form-title">Stores</h2>
                <?php
                $store_terms = get_terms('store_type', array('hide_empty' => false)); 
                foreach ($store_terms as $term) {
                    echo '<div class="input-group">';
                    echo '<input type="checkbox" name="store[]" value="' . esc_attr($term->slug) . '" id="store' . esc_attr($term->term_id) . '" class="styled-checkbox"><label for="store' . esc_attr($term->term_id) . '">' . esc_html($term->name) . '</label>';
                    echo '</div>';
                }
                ?>
            </div>
            <div id="posts-per-page-filter" class="hidden">
                <h2 class="form-title">Posts Per Page</h2>
                <select id="postsPerPage" name="postsPerPage" class="selector">
                    <option value="10">10</option>
                    <option value="30">25</option>
                    <option value="50">50</option>
                    <option value="100">100</option>
                </select>
            </div>
            <div id="sort-filter" class="hidden">
                <div class="input-group">
                    <h2 class="form-title">Sort By</h2>
                    <select id="sortOrder" name="sortOrder" class="selector">
                        <option value="price-low-to-high">Price: Low to High</option>
                        <option value="price-high-to-low">Price: High to Low</option>
                        <option value="discount-low-to-high">Discount: Low to High</option>
                        <option value="discount-high-to-low">Discount: High to Low</option>
                        <option value="date-newest" selected>Date: Newest</option>
                        <option value="date-oldest">Date: Oldest</option>
                    </select>
                </div>
            </div>

        </form>

This below is the query builder:

function build_query_args($params, $is_initial_load = false) {

    $args = array(
        'post_type' => 'post',
        'posts_per_page' => isset($params['postsPerPage']) ? intval($params['postsPerPage']) : 10,
        'paged' => isset($params['page']) ? intval($params['page']) : 1,
    );

    if (!empty($params['s'])) {
        $args['s'] = sanitize_text_field($params['s']);
    }

    $tax_query = array('relation' => 'AND');

    if (!empty($params['category'])) {
        $tax_query[] = array(
            'taxonomy' => 'product_categories',
            'field'    => 'slug',
            'terms'    => $params['category'],
            'operator' => 'IN'
        );
    }

    if ($is_initial_load || empty($params['status'])) {
        $tax_query[] = array(
            'taxonomy' => 'category',
            'field' => 'slug',
            'terms' => 'active-deals',
        );
    } elseif (!empty($params['status'])) {
        $tax_query[] = array(
            'taxonomy' => 'category',
            'field' => 'slug',
            'terms' => $params['status'],
        );
    }

    if (!empty($params['store'])) {
        $tax_query[] = array(
            'taxonomy' => 'store_type',
            'field' => 'slug',
            'terms' => $params['store'],
        );
    }

    if (!empty($tax_query)) {
        $args['tax_query'] = $tax_query;
    }

    $discount_percentage = $is_initial_load ? 0 : (isset($params['discount']) ? intval($params['discount']) : 0);
    $args['meta_query'] = array(
        array(
            'key' => '_discount_percentage',
            'value' => $discount_percentage,
            'type' => 'NUMERIC',
            'compare' => '>=',
        )
    );

    if (!empty($params['sortOrder'])) {
        switch ($params['sortOrder']) {
            case 'price-low-to-high':
                $args['meta_key'] = '_discount_price';
                $args['orderby'] = 'meta_value_num';
                $args['order'] = 'ASC';
                break;
            case 'price-high-to-low':
                $args['meta_key'] = '_discount_price';
                $args['orderby'] = 'meta_value_num';
                $args['order'] = 'DESC';
                break;
            case 'discount-low-to-high':
                $args['meta_key'] = '_discount_percentage';
                $args['orderby'] = 'meta_value_num';
                $args['order'] = 'ASC';
                break;
            case 'discount-high-to-low':
                $args['meta_key'] = '_discount_percentage';
                $args['orderby'] = 'meta_value_num';
                $args['order'] = 'DESC';
                break;
            case 'date-newest':
                $args['orderby'] = 'date';
                $args['order'] = 'DESC';
                break;
            case 'date-oldest':
                $args['orderby'] = 'date';
                $args['order'] = 'ASC';
                break;
        }
    }

    return $args;
}

the processing method:

function ajax_filter_posts_handler() {
    check_ajax_referer('filter_posts_nonce', 'nonce');
        
    $is_initial_load = isset($_POST['initial_load']) && $_POST['initial_load'] === 'true';
    $args = build_query_args($_POST, $is_initial_load);
    

    $query = new WP_Query($args);


    ob_start();
    if ($query->have_posts()) {
        while ($query->have_posts()) {
            echo do_shortcode('[elementor-template id="71260"]');
        }
    } else {
        error_log('No posts found');
        echo '<h2>No posts found.</h2>';
    }
    $content = ob_get_clean();

    $posts_per_page = $args['posts_per_page'];
    $total_posts = $query->found_posts;
    $total_pages = ceil($total_posts / $posts_per_page);

    $pagination = generate_custom_pagination($args['paged'], $total_pages);

    wp_reset_postdata();

    $response = array(
        'content' => $content,
        'pagination' => $pagination,
        'total_posts' => $total_posts,
        'total_pages' => $total_pages,
        'current_page' => $args['paged'],
        'posts_per_page' => $posts_per_page
    );
    
    wp_send_json_success($response);
}

and the AJAX call…

function filterPosts(page = 1) {
            let status = $('[name="status[]"]:checked').map(function() { return $(this).val(); }).get();
            
            if (status.length === 0 && isInitialLoad) {
                status = ['active-deals'];
                $('#statusActive').prop('checked', true);
            }
        
            let data = {
                'action': 'filter_posts',
                'nonce': ajax_object.nonce,
                's': $('.search-input').val(),
                'category': $('[name="category[]"]:checked').map(function() { return $(this).val(); }).get(),
                'status': status,
                'store': $('[name="store[]"]:checked').map(function() { return $(this).val(); }).get(),
                'discount': $('#discountRange').val(),
                'sortOrder': $('#sortOrder').val(),
                'postsPerPage': $('#postsPerPage').val(),
                'page': page,
                'initial_load': isInitialLoad
            };
            
            
            $.ajax({
                url: ajax_object.ajax_url,
                type: 'POST',
                data: data,
                success: function(response) {
                    console.log('AJAX response:', response);
                    if (response.success) {
                        $('.elementor-loop-container').html(response.data.content);
                        updatePagination(response.data.pagination);
                        updateTotalPosts(response.data.total_posts, response.data.total_pages, page);
                        $('html, body').animate({
                            scrollTop: $('.elementor-loop-container').offset().top
                        }, 500);
        
                        updateUrlParameter('paged', page);
                    } else {
                        console.error('Error in AJAX response');
                    }
                },
                error: function(xhr, status, error) {
                    console.error('AJAX error:', error);
                },
                complete: function() {
                    isInitialLoad = false;
                }
            });
        }

        function updatePagination(paginationHtml) {
            $('.elementor-pagination').html(paginationHtml);
        }

        function updateTotalPosts(totalPosts, totalPages, currentPage) {
            $('.total-posts').text(totalPosts);
            $('.total-pages').text(totalPages);
            $('.current-page').text(currentPage);
        }

        function updateUrlParameter(key, value) {
            var url = window.location.href;
            var re = new RegExp("([?&])" + key + "=.*?(&|$)", "i");
            var separator = url.indexOf('?') !== -1 ? "&" : "?";
            if (url.match(re)) {
                url = url.replace(re, '$1' + key + "=" + value + '$2');
            }
            else {
                url = url + separator + key + "=" + value;
            }
            
            url = url.replace(/&e-page-[^=]+=[^&]*/g, '');
            window.history.pushState(null, null, url);
        }

        $('.searchandfilter input, .searchandfilter select').on('change', function() {
            filterPosts(1);  
        });

        $('#postsPerPage').on('change', function() {
            filterPosts(1);  
        });
        
        $('#sortOrder').on('change', function() {
            filterPosts(1);
        });

        $(document).on('click', '.elementor-pagination a, .page-numbers a', function(e) {
            e.preventDefault();
            let currentPage = parseInt($('.current-page').text()) || 1;
            let page;

            if ($(this).hasClass('prev') || $(this).text().includes('Previous')) {
                page = currentPage - 1;
            } else if ($(this).hasClass('next') || $(this).text().includes('Next')) {
                page = currentPage + 1;
            } else {
                page = parseInt($(this).text());
            }

            if (isNaN(page) || page < 1) {
                page = 1;
            }

            filterPosts(page);
        });

        filterPosts(1);
    }

    $(document).ready(function() {
        if ($('body').hasClass('category-2') || $('body').hasClass('page-id-161') || $('body').hasClass('category-515')) {
            initializeFiltering();
        }
    });