Not really a problem, just an oddity. The gump for tinkering gives me an indication that my GM quality items have been given my maker's mark, even when it doesn't. In my most recent experience, with shovels, they get their 100 uses of durability but are otherwise unmarked. I'm guessing it just needs a more appropriate message in the gump for that to just say I crafted an exceptional quality item, sans the maker's mark application. I don't think tinkered items should have maker's marks in general and I'm not even sure if any really do.