Nothing has changed about the item since the hak update. The spell Knock worked that way looong before the update, only the spell description was altered with the update to say so. The Rokuma Bone Key has worked this way as long as the spell has worked this way as well, so the update didn't do anything to the item, and they are actually useful to low level rogues who struggle with opening some locks.
Now, that doesn't mean I'm opposed to the spell it casts being cast at a higher level, I've often thought that it being cast with only the +1 was a bit strange, but I don't think anything above a +3 should be done personally. There's another item out there that's the same thing but with 50 charges and having it give more than +3 to open lock doesn't seem like a very balanced decision to me. Consider also the fact that increasing the level the spell is cast at will increase the value of the item and mean that it will not be found so often by lower levels who will get the most use out of it and only make it even more costly still than the thieves tools you compared them to