I think the issue is that the Battle Team is still registered as being used for that old tournament, and with the online services shut down there may not be a normal way to clear it in-game.
If you’re comfortable editing the save, PKHeX might be worth checking for a Battle Team/competition-related flag. I’d definitely keep a backup of the original save before changing anything, though. Hopefully someone familiar with the Gen 7 save structure can take a look at your file and point you to the exact flag that needs clearing. sports games