The function _acf_form_register_fields and the call in acf_form_head() was added 22 days ago… in version 5.3.8, the last version you say it was working the fields were defined during validation process and now they are defined before the validation process. Basically, before the fields where defined just before they where possibly used and now they are defined earlier process. I don’t know if this is a bug or a feature, but basically now if acf_form_head() is not called the local fields for _post_title and _post_content will not be defined unless you call the new function I posted above.
