These days, one's allegiance is determined by one's estate. If one has no estate (and is not a Lord or Duke), one cannot be in a Duchy.
If someone without an estate is showing up as being in the Duchy of Fissoa, I believe that is itself a bug, regardless of how they got to that state.
Sounds like a bug to me given what I have from the player in question. I'll post in bug tracker.
For what it's worth, I can understand how allegiance is tied to estates, but there ought to be a way to re-install a lord who was just kicked from their estate. As it stands the former Lord has to take an estate in Duchy Drowenton before being reassigned. If there were no estates available in the duchy, it would require someone making one simply for the purpose of allowing the character to change allegiance.
From an RP perspective, I would think that it would make sense that actively taking an estate would change one's allegiance, but involuntarily losing an estate would not.
