I think you misunderstand. I'm not saying every single Duke should get furious, call the lord nine different curses, demand a duel and never speak to the lord again, but they should say something, even if it's just to politely ask for an explanation. Unless the region switch was part of some arrangement he made with the other duke it really should not go without a response.
A duke saying absolutely nothing when a region leaves his Duchy should be as rare as a King not saying anything when a region or duchy leaves his realm.
