So I have a simple WordPress plugin that just allows me to use custom page templates. Here’s the code:

my-special-template.php

<?php
/**
 * Plugin Name: My Special Template
 * Description: Awesome description.
 **/

function my_template_array()
{
    $temps = [];

    $temps['john.php'] = 'John';
    $temps['jack.php'] = 'Jack';

    return $temps;
}

function my_template_register($page_templates,$theme,$post)
{
    $templates = my_template_array();

    foreach ($templates as $tk => $tv)
    {
        $page_templates[$tk] = $tv; 
    }

    return $page_templates;
}
add_filter('theme_page_templates','my_template_register',10,3);

function my_template_select($template)
{
    global $post, $wp_query, $wpdb;

    if (isset($post->ID))
    {
        $page_temp_slug = get_page_template_slug($post->ID);

        $templates = my_template_array();

        if (isset($templates[$page_temp_slug]))
        {
            $template = plugin_dir_path(__FILE__).'templates/'.$page_temp_slug;
        }
    }

    //echo '<pre>Preformatted:';print_r($page_temp_slug);echo '</pre>';

    return $template;
}
add_filter('template_include', 'my_template_select', 99)

?>

And the templates:

<?php
/* Template Name: Jack */
?>
Jack

<?php
/* Template Name: John */
?>
John

When I use a theme like Hello Elementor, or Simple Persona, the custom templates can be selected from the template dropdown and the plugin works perfectly:

However, if I used a theme like Twenty Twenty Four that comes pre-installed with WordPress, then I get this:

I can’t actually select my plugin templates. What gives? How can I fix this? My WordPress is 6.6.1, or the latest one at this moment.