Even if the “Allow members to leave the group freely” option is disabled. It is possible for group owners to remove themselves from their own group. This makes it possible to lock themselves out.
I don’t really consider that a bug and more so that the king has the ability to self-exile.
In this case, there should be a warning dialogue stating that you are removing yourself and you may not regain your ownership privileges.
That is not a bad suggestion. Though, how often do your group owners remove themselves from their own group?
Not very often, but I do have cases where group owners would accidently remove themselves.