I recently imported from vBulletin 3.8. There was a banned member for 3 days (from vBulletin, banned for reaching a certain infractions threshold). On XenForo, the user was unbanned after the 3 days (banned - old value: yes / new value: no) however he wasn't moved back to the registered usergroup. On vBulletin all banned people were moved to a banned usergroup. On XenForo I have also configured a banned usergroup and set it properly on "Add User Group on Ban" (it's the same banned usergroup imported from vBulletin).
I'm not sure if this problem happens only with migrated data or also could happen on a stock XF.