It is a bug, other users are now reporting it. Seems like a problem with the offseason processing was triggering a partial data rollback, affecting even set tactics. I think I fixed it now but I can't be sure, we need time to see if it happens again. Until then, I added a new warning update to the offseason processing text on the main news page saying basically the same thing I already wrote here (default lineups and doublechecking).
All that said, I cannot rerun affected games, sorry to say. Hopefully it won't happen again and the damage will remain but be minimized.