It could be related to the revision thing. If a revision isn’t being saved then is there a place to record who made the change?
You might want to submit a new support ticket https://support.advancedcustomfields.com/forums/topic/_edit_last-not-being-updated-on-when-using-frontend/
You might also want to look into adding an acf/save_post filter and make this update yourself if it’s required.
