Hi falconmick,
I had the same issue with my Custom Post Order for a custom post type.
I managed to get custom ordering working by adding support for ‘page-attributes’ when I registered my custom post type. Something like this:
'supports' => array( 'title', 'taxonomy', 'revisions', 'page-attributes' )
After that, I just had to add order=asc to my rest url so that it wasn’t in reverse/descending custom order.
I hope that makes sense.
