Confirming that this also happenned to me after i updated to v5.4 of ACF with the latest version of WP 4.5.3. In my setup, this specifically happens when the function wp_update_post() is called on a form submitted from the front-end. However when i revert back to my previous ACF version 5.3.9.2 …. all works fine and no issues. Hence i guess only when the upgrade happens.
Hoping this can be resolved at the earliest.
Thanks,
Chris
