This seems to finally have been fixed in ACF Pro Version 5.2.0, for the most part. Send a never-valid ID as the post_id for new posts (like -5) and then check for the when you save the post. You’ll need to manually save the post, or add to the acf/pre_save_post filter as described here: http://www.advancedcustomfields.com/resources/using-acf_form-to-create-a-new-post/
It doesn’t work when sending a string as the post_id still using acf_form, but when editing you’re sending an int so it seems to work.
