Show meta box only for top parent page ID

Hi again,

I found the problem for your situation. Your check doesn't run when post is updated, because there's no $GET['post']. In that case, you need to get post ID via $_POST. Here is the code:

function prefix_parent_page_id_check() {
	$parents = get_post_ancestors( prefix_get_post_id() );
	return $parents && 901 == array_pop( $parents );
}

function prefix_get_post_id() {
	if ( isset( $_GET['post'] ) ) {
		return intval( $_GET['post'] );
	}
	if ( isset( $_POST['post_ID'] ) ) {
		return intval( $_POST['post_ID'] );
	}

	return false;
}