acf/load_field not working?

Hey, I would like to use acf/load_field filter to prefill a field (for a auto increment number on a special post type).

But it seems nothing happens.

I tried this:

add_filter('acf/load_field/name=untertitel', 'fill_field');
function fill_field( $field ) {

    $field['value']='Test';
	$field['readonly'] = true;
    return $field;
}

Nothing happens with the field. It stays blank.

Any ideas?