When post is saved from Gutenberg with meta, it causes two requests — first through REST API and second to post.php (It’s why by a redirect browser is getting a whole admin page with message about JavaScript) and it’s only sending meta boxes, so no main fields are presented in request.
I manage to prevent rewrite be second request by your hook ‘sublanguage_admin_update_post’, but for custom posts it still don’t work. Even if you have function to ?register post types for Parse meta for REST? and seem to cover everything. I solved this by remove REST API for this custom post type. It shut Gutenberg down to it, but it can stay this way.
I still will be glad to some tips, so ))
And thank you any way!
