You issue probably comes down to the issue of using the ‘save_post’ hook. ACF fields are saved on this hook with a priority of 10. Your filter also runs on the same hook with a priority of 10. If your action is called before the acf action is called then the acf fields have not been saved yet. This will cause errors on new posts because all of the get_field() calls will return NULL.
Increase the priority or your add_action() call to > 10
