Validation happens on the server yes, but it is fired using JS. ACF intercepts the form submission using JS, it sends an AJAX request to validate, if validation fails ACF prevents the post from being saved. It is the last part that is a problem. There is nothing in the new editor that can prevent it from saving, there is nothing that will stop it from saving, at least this is my understanding. Beyond that you’d need to talk directly with Elliot.
