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 18.104.22.168 …. all works fine and no issues. Hence i guess only when the upgrade happens.
Hoping this can be resolved at the earliest.
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