Thanks for the report!
Fair point — that's undocumented and it shouldn't be. Blacksmith's Help only works on the first two weapons, 11★–15★. After you promote past those, it's gone for good. It's training wheels for your first danger zone. The real progression is Rebirth: you lose everything except Luck(and some consumables), and Luck permanently multiplies your enhance rates. That's what carries you past the early walls. Fixing the explanation in the next patch.