flipping duchy allegiance

[fodder]

we saw something interesting on bt the other day, a region can flip duchy if the lord is physically in a region of the target duchy

You can switch to other duchies on two conditions: One, at least one region of that duchy is a neighbour of your region, or two, you are in a region of the duchy. This means you can switch to any duchy you desire simply by travelling there and pledging your allegiance.
If the duchy you switch your region to belongs to a different realm, you and all your knights will automatically join that realm.

which begs the question... what do you do if the target duchy has no regions?

[Indirik]

Simple: You don't switch to it.

And yes, this is intentional.