get_posts() returns empty on custom post type /wp-admin/edit.php

I need to do a get_posts() to fetch a list of "product" type posts, but when I do it on edit.php?post_type=mycustompost it returns empty. If I just simply do the same function but on edit.php (posts index page) it returns properly.

add_action('admin_head', 'custom_admin_head');

function custom_admin_head() {
    $args = array(
        'post_type'      => 'product',
        'posts_per_page' => 30,
    );
    $products = get_posts($args);

In this example

on edit.php?post_type=mycustompost… $products is empty

on edit.php… $products has 30 entries

How do I get it to return all 30 entries on the custom post type edit.php in wp-admin?