On 8.1 (or 8.1.1) if you specify the user boot code ("mc user=qopit") you get this during the boot:
chpasswd: unknown user qopit
chown: unknown user qopit
chown: unknown user/group qopit:staff
chown: unknown user/group qopit:staff
chown: unknown user/group qopit:staff
chown: unknown user/group qopit:staff
Then you are prompted for a password. When you can't enter that you get the normal box login and the tc user exists and can login. Unsurprisingly, the specified user does not exist on the system after that.
Core 8.0 and earlier work fine. The problem was introduced in 8.1.