I for one have never seen it. In Iashalur several regions switched from Gaston to Niselur duchy and Duke Qadan never said a word to me or as far as I know to the Lords who were abandoning him.
A duke absolutely should publicly rebuke a lord who's abandoning his duchy and stealing his region. That's at minimum. Half the time it should end in a duel if not an ongoing feud between the two duchies.
