I just ran into this issue whereby filtering a post_object’s query wasn’t taking, but it looks like when a post_object’s type is ‘page’, ACF is effectively firing get_pages vs. creating a new WP_Query, so the args are all different.
So, running this choked:
$args['post_parent'] = $parent->ID;
$args['orderby'] = 'menu_order';
While running this works great:
$args['child_of'] = $parent->ID;
$args['sort_column'] = 'menu_order';
Hopefully this will help anyone else who’s wondering why this isn’t working properly.