ACF – How to set the menu order for the relationship field

orderby defaults to post_date, to order by the IDs you supply, you have to orderby post__in

$ids = get_field('homepage_lineup_new', 'option', false, false); 

$query = new WP_Query(array(
'post_type'         => 'post',
'posts_per_page'    => 15,
'post__in'          => $ids
'orderby'           => 'post__in'
));