I was at war with a nation and their vassal. I occupied cities from both the nation and the vassal. During that war, the vassal stopped being the vassal of the nation I was at war with.

I don't know what the expected behaviour is for cities under these circumstances, but I retained control of the ex-vassal's city in an occupied state. When the war eventually finished, the ex-vassal's occupied city stayed in the occupied state despite there being no current war.

This is the city many turns later, still occupied despite being at peace.

The attached game state is well after this has occurred as I didn't think to do a bug report at the time.

auto-save 1255.ctr

auto-save 1256.ctr

Diagnostics (2021'08'20 @0759'54''618''').html

This bug is hard to reproduce because the steps aren't under player control. But it appears to be an issue in the logic that handles what happens to occupied cities of 3rd parties during wars that cease to be part of the war.