I've been thinking and I have a theory what is causing this game breaking bug.The files affected all seem to have beaten 10-man smash with all the pre-existing characters before they downloaded Mewtwo, so the game recorded that they had beaten 10-man smash with everyone. Then when suddenly they're beating 10-man smash with the DLC character, a new character, it doesn't compute with the game save which thinks you've already beaten 10-man smash with all the characters. The game may then think 'we have a cheater' and implement the anti-cheating tricks.It's only a theory, but either way it's due to programming oversights with the patch and Mewtwo.
But why only 10-Man Smash?
My guess is that they meant to apply checks to all the features to prevent such a glitch from happening but the coding for the check for 10-man smash is faulty.
Huh. I wonder if it has to do with the challenge where you have to beat it within 25 seconds with everyone?
I've done some game programming, and yeah, that sounds pretty accurate. It looks like they didn't plan for DLC characters until later in development, so some parts of the game were created before factoring that in and flew under the radar when making changes for the DLC. Fixing the bug should be easy and I imagine we'll get a patch within the next couple of days, but actually fixing the corrupted data and recovering previous results might take some more time.
Knowing Nintendo, we may not get a patch until the patch can also fix the affected games. For evidence I point to Pokemon X and Y. When the Lumiose city saving bug was discovered Nintendo made sure the patch would also fix files affected by the glitch before they distributed it.
True enough, but I don't think that's the best strategy. When solving a leaky pipe, do you patch the hole first or clean up the water?
I agree with you, the patch should come first, but with Nintendo's history they're going to try and fix the patch and recover data in one swoop.