We should only deliver Blu-ray from our Core, not Blu-ray Disc at all (we did in the past though). So good catch. I think something went wrong there in the box set add area indeed. I’ve reported the issue for fixing. Easily reproduced as well!
The fix which you probably know about is to go to Manage Pick List, format list, and merge them to Blu-ray and you’ll be good.
Or wait until you’ve added everything and THEN merge them because the problem will return until we fix it.