I suppose Castillo shouldn't have been stopped against Corrales either

I didn't think it was that bad, Stevens pulled it out of the fire no doubt and was taking a beating until then, but he landed some finishers. If a fighter is truly out on the ropes with punches incoming, you can never call it a bad stoppage imo. If a ref hesitates in those spots you can end up with a Mercer-Morrison type finish.