The filter that you are trying to use only works in the admin when selecting posts, it does nothing on the front end of the site. On the front end ACF always orders the posts in the order select. If you want to have a different order then you need to do your own query on the posts. Get the field without formatting and then use that in the post__in argument for WP_Query().
// get unformatted value
// will return an array of post IDs
$post__in = get_field('post_object_field', false, false);
