If a registered member is a member of a single group and that group is NOT the Registered Users group (as would always be the case with the Registered COPPA users group, although it happens no matter what the group is) and you remove that member from that group in the ACP, the users table's group_id field for that user is automatically changed to the group id of the Registered users group, which would be expected.

However, the user_id is removed from the user_group table and no new entry is made, with the result that the user appears in no groups in the ACP and UCP and retains the permissions of the group he was removed from.

Perhaps this is intended behavior to prevent, for example, people inadvertently removed from Registered COPPA users getting Registered users permissions. But then it would be preferable to merely delete the group_id entry in the users table, which would have the same effect of their retaining their permissions, but the advantage that it would be easy to see what users were not effectively in any group with a simple query and fix it with a simple query instead of something like:

INSERT INTO phpbb_user_group (group_id, user_id, group_leader, user_pending) SELECT 2, user_id, 0, 0 FROM phpbb_users WHERE group_id = 2 AND user_id NOT IN(SELECT user_id FROM phpbb_user_group WHERE group_id = 2);