Display ACF if parent page has specific template

For anyone dealing with the same issue, I just needed to change:

$choices[ $template_name ] = $template_name;

with:

$choices[ $template_filename ] = $template_name;

Consider a page template Homepage (page-home.php). This way the template name Homepage will appear on the custom fields page but $rule['value'] will actually return page-home.php and then we can compare this with get_page_template_slug( $post->post_parent ).