This issue https://support.advancedcustomfields.com/forums/topic/google-map-not-displaying-on-wp-backend/ has been open for nearly 3 years and is marked “solving”. It is however closed to new replies.
As far as I can tell, adding an API key through the methods suggested does not add the API key to the request to googleapis. It has nothing to do with billing or new versions of Google Maps or the validity of the key: the key is simply never attached to the request.
What is the status of the fix?