I recently observed the same problem when I was using acf_form() with the
'post_id' => 'new_post' setting.
For me, the solution was to first create an
'auto-draft' post and then run the acf_form() on this posts’s ID:
$thePostId = wp_insert_post( array('post_title' => 'Temporary Post Title','post_status' => 'auto-draft') );
'post_id' => $thePostId,