If i recall, at the time of migration, we were informed that accounts would need to be “activated” on the new forums by clicking through an email or something similar, esp. for accounts that had some other issue. I forget the exact details, but I’m sure @wumpus could provide more.
Anyway, point is, if your account pointed to a dead email, or if you just ignored it and never tried to login at all, it never fully became live on the new system. Posts were preserved, but you never gain a full profile or whatever, I guess.