I encountered the very same issue. After a few hours, I decided to try to use the second function mentioned on https://www.advancedcustomfields.com/resources/google-map/ (the one which is not suggested for ACFPRO useres):
function my_acf_google_map_api( $api ){
$api['key'] = 'xxx';
return $api;
}
add_filter('acf/fields/google_map/api', 'my_acf_google_map_api');
… and it works. I have no idea if (with future updates) this may result in issues on the long run, but nevertheless, it’s a solution for now.
