OK here are more information to help for the debugging.
From what I can see on network capture, the ACF datas aren’t sent to the server at the first post save, except for the fields validation.
So basically, the network activity that I see when I push the publish button is something like:
Client :
-POST /wp-admin/admin-ajax.php …. action=acf/validate_save_post (the ACF fields are here)
Server :
-Ok valid, success
Client :
-POST /index.php/wp-json/wp/v2/posts/<id>?_locale=user (in data, there are the post title, categories, status, but no ACF fields…)
Server :
-OK, post created
Client :
POST /wp-admin/admin-ajax.php action=acf/ajax/check_screen
Server :
-Some informations
And that’s all. In this configuration, the ACF data are never sent to the server, except for the validation. So it can be a beginning of the investigation here.
