I created a small plugin where users can subscribe to authors, but for some reason users are not notified when the author publishes a new post. Please help to solve this problem. I checked the logs, it shows that the notification was sent, but the notification itself is not there!

<?php
/*
Plugin Name: Author Subscription and Notification
Description: Allows users to subscribe to authors and receive notifications when authors publish new posts.
Version: 1.3
Author: test
*/

// Hook to create the database table upon plugin activation
register_activation_hook(__FILE__, 'create_subscriptions_table');

function create_subscriptions_table() {
    global $wpdb;
    $table_name = $wpdb->prefix . 'author_subscriptions';
    $charset_collate = $wpdb->get_charset_collate();

    $sql = "CREATE TABLE $table_name (
        id bigint(20) NOT NULL AUTO_INCREMENT,
        user_id bigint(20) NOT NULL,
        author_id bigint(20) NOT NULL,
        PRIMARY KEY (id),
        UNIQUE KEY user_author (user_id, author_id)
    ) $charset_collate;";

    require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
    dbDelta($sql);
}

// Hook to add subscription button in the post content
add_filter('the_content', 'add_author_subscription_button_to_post');

function add_author_subscription_button_to_post($content) {
    if (is_single() && is_user_logged_in()) {
        $author_id = get_the_author_meta('ID');
        $current_user_id = get_current_user_id();
        
        global $wpdb;
        $table_name = $wpdb->prefix . 'author_subscriptions';

        // Check if user is already subscribed
        $is_subscribed = $wpdb->get_var($wpdb->prepare(
            "SELECT COUNT(*) FROM $table_name WHERE user_id = %d AND author_id = %d",
            $current_user_id, $author_id
        ));

        $button_text = $is_subscribed ? 'Unsubscribe' : 'Subscribe';
        $button_html = '<button id="subscribe-author" class="subscribe-button" data-author-id="' . esc_attr($author_id) . '">' . esc_html($button_text) . '</button>';
        $button_html .= '<div id="subscription-message"></div>';
        
        $content .= $button_html;
    }
    return $content;
}

// Enqueue the JavaScript file and localize script
add_action('wp_enqueue_scripts', 'enqueue_subscription_script');

function enqueue_subscription_script() {
    if (is_single() && is_user_logged_in()) {
        wp_enqueue_script('subscription-script', plugin_dir_url(__FILE__) . 'subscription.js', array('jquery'), '1.0', true);
        wp_localize_script('subscription-script', 'subscriptionData', array(
            'ajax_url' => admin_url('admin-ajax.php'),
            'nonce'    => wp_create_nonce('subscription_nonce')
        ));
    }
}

// Handle the AJAX request for subscription
add_action('wp_ajax_handle_author_subscription', 'handle_author_subscription');

function handle_author_subscription() {
    check_ajax_referer('subscription_nonce', 'nonce');

    if (is_user_logged_in()) {
        global $wpdb;
        $table_name = $wpdb->prefix . 'author_subscriptions';

        $current_user_id = get_current_user_id();
        $author_id = intval($_POST['author_id']);
        $operation = sanitize_text_field($_POST['operation']);
        
        if ($operation === 'subscribe') {
            $wpdb->insert($table_name, array(
                'user_id' => $current_user_id,
                'author_id' => $author_id
            ));
            wp_send_json_success('Subscribed successfully.');
            error_log("User ID $current_user_id subscribed to author ID $author_id");
        } else if ($operation === 'unsubscribe') {
            $wpdb->delete($table_name, array(
                'user_id' => $current_user_id,
                'author_id' => $author_id
            ));
            wp_send_json_success('Unsubscribed successfully.');
            error_log("User ID $current_user_id unsubscribed from author ID $author_id");
        }
    }
    wp_send_json_error('Failed to update subscription.');
}

// Hook to send notifications when an author publishes a new post
add_action('publish_post', 'notify_subscribers_on_new_post');

function notify_subscribers_on_new_post($post_id) {
    global $wpdb;
    $table_name = $wpdb->prefix . 'author_subscriptions';

    $post = get_post($post_id);
    $author_id = $post->post_author;

    error_log("New post published by author ID: " . $author_id);

    // Get subscribers
    $subscribers = $wpdb->get_results($wpdb->prepare(
        "SELECT user_id FROM $table_name WHERE author_id = %d",
        $author_id
    ));

    error_log("Subscribers found: " . count($subscribers));

    foreach ($subscribers as $subscriber) {
        // Send BuddyPress notification
        bp_notifications_add_notification(array(
            'user_id'           => $subscriber->user_id,
            'item_id'           => $post_id,
            'secondary_item_id' => $author_id,
            'component_name'    => 'buddypress',
            'component_action'  => 'new_post_by_subscribed_author',
            'date_notified'     => bp_core_current_time(),
            'is_new'            => true,
        ));
        error_log("Notification sent to user ID: " . $subscriber->user_id);
    }
}

// Custom notification format
add_filter('bp_notifications_get_notifications_for_user', 'custom_bp_notification_format', 10, 5);

function custom_bp_notification_format($content, $item_id, $secondary_item_id, $action, $component_name) {
    if ($component_name === 'buddypress' && $action === 'new_post_by_subscribed_author') {
        $author_name = get_the_author_meta('display_name', $secondary_item_id);
        $post_title = get_the_title($item_id);
        $post_url = get_permalink($item_id);
        
        $content = sprintf(__('New post by %s: <a href="%s">%s</a>', 'text-domain'), $author_name, $post_url, $post_title);
    }
    
    return $content;
}
?>

Here is the log itself!

[23-May-2024 09:42:15 UTC] a:0:{}
[23-May-2024 09:42:40 UTC] New post published by author ID: 1
[23-May-2024 09:42:40 UTC] Subscribers found: 2
[23-May-2024 09:42:40 UTC] Notification sent to user ID: 1
[23-May-2024 09:42:40 UTC] Notification sent to user ID: 27
[23-May-2024 09:42:43 UTC] a:0:{}