Hook from plugin doesn't fire up from external PHP script

I have this habit of installing a plugin with all my custom hooks, instead of writing a functions.php file with the same PHP code in it. That way I get the same functionality without the need of installing a child theme...

So my plugin looks like this:

?php
/**
 * Plugin Name: Custom Code
 * Description: Contains custom code needed by the website
 * Version:     1.1.5
 * Author:      Me
 * License:     GPL2
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 * Text Domain: all-custom-code
 **/

if (!defined('WPINC')) {
    die;
}

add_action('pmxi_gallery_image', 'set_media_category_to_images', 10, 4);
function set_media_category_to_images($post_id, $att_id, $filepath, $is_keep_existing_images = '')
{
   // Code here...
}

add_action('pmxi_after_xml_import', 'count_product_images', 10, 2);
function count_product_images($import_id, $import)
{
   // Code here...
}

add_filter('wp_unique_term_slug', 'prevent_subcat_sufix', 10, 3);
function prevent_subcat_sufix($slug, $term, $original_slug)
{
   // Code here...
}

add_filter('woocommerce_ajax_variation_threshold', 'set_variations_loading_threshold', 10, 2);
function set_variations_loading_threshold($amount, $product)
{
   // Code here...
}

In this particular website I have an action hooked to after_delete_post which looks like this:

add_action('after_delete_post', 'clean_pmxi_tables', 20, 2);
function clean_pmxi_tables($postid, $post)
{
    global $wpdb;

    $wpdb-query(DELETE FROM {$wpdb-prefix}pmxi_hash WHERE post_id NOT IN (SELECT ID FROM {$wpdb-prefix}posts WHERE post_type = 'product' OR post_type = 'product_variation'););
    $wpdb-query(DELETE FROM {$wpdb-prefix}pmxi_posts WHERE post_id NOT IN (SELECT ID FROM {$wpdb-prefix}posts WHERE post_type = 'product' OR post_type = 'product_variation'););
    $wpdb-query(DELETE FROM {$wpdb-prefix}pmxi_images WHERE attachment_id NOT IN (SELECT ID FROM {$wpdb-prefix}posts WHERE post_type = 'attachment' AND post_parent IN (SELECT ID FROM {$wpdb-prefix}posts WHERE post_type = 'product' OR post_type = 'product_variation')););
}

I also have an external script in a separate file, which however loads the WP core (with require_once '../wp-load.php';), which among other operations, deletes some attachments using this code:

if (isset($_GET['id'])) {
    if ($image = get_post(intval($_GET['id']))) {
        wp_delete_post($image-ID, true);
    }
}

So, one would expect the hook defined in my plugin to fire up. But I noticed it doesn't, as I compared the rows in the affected tables and they are the same.

For control, I even put a file_put_contents(ABSPATH . '/TTTT.txt', time() . \r\n, FILE_APPEND); in my after_delete_post action, and still no file was written. So I concluded that it doesn't fire up at all... I played with priorities a little bit, changing 10 to 20, 99, 5, but nothing changed...

Now my suspicion fell on hooks not firing up through actions of external scripts in general, but I can't find any resource claiming this, nor can I find any resource to fix that behavior (supposing my suspicion is correct).

Anyone can shed some light on this? TIA. :D