In Zambia, the eastern vanilla state has been shrunk into Northern Nyasaland. The game's 'desired' border though (marked with a yellow dash line) is still vanilla and goes into the Zambian state. I thought this was just a visual issue until Zambia had a revolt in half of the region. When they retook the state, the revolt's land was folded into Kazemba, making a split-state and changing the states borders.

I have never seen this happen before so I'm struggling to describe it. It seems if you have a 'desired' border that's different from the state border and a split-state appears in the state, either by a revolt or treaty-port, then the new split-state will be apart of the 'desired' region instead of the state it revolted from.

So if a one-state country has a revolt and it overlaps with a 'desired' border, then the state border permanently changes.