The 12k should be smooth like your 5 k.
3 people on this page other than you have had similar problems so we're just all lucky. :)
If it's only in isolated spots you could just use the stone & wait until regular lapping removes the problem. My one was faulty only on one edge so I just turned it round. Eventually it smoothed out. You should mark your stone & set a limit on how much you are prepared to lap tho. Sometimes a product warrants returning.