If the region had reverted back to its previous duchy, that would have caused no problem. The issue here is that it reverted to the closest contiguous duchy instead of going for historical links. I understand that would probably be hard to code though.
At the moment, I've entered the Lordhip election, crossing my finger that this will move the region in my duchy and not drop the Duke title. Let's see.
Try not to lose all of your duchy's regions... Had you swayed the lord of the region adjacent to the one you wanted back to switch to you duchy before the TO, the region would be yours to do what you want with.
This is obviously not a fix, but it could prevent such frustration scenarios.
