yes, this actually happend:

i attacked and took a city form an empire (me kingdom). a few turns later, the army of the enemy shows up, takes the city back but also takes some massive losses. so i immediatly counterattack on my turn and kill the enemy sovereign, who then died defending in his city, because it was mine for some turns and didnt turn back to fallen land?

this his dead should not have happned. either the terrain changed to fast after i conquered the city or it should revert back to fallen faster.

also, it looks like fallen city need more population to level up, which means you need those fallen houses to not waste huge amount of food on it. so leveling up fallen city as kingdoms is a pain. the other way around it may be too easy for leveling. something needs to change here, the amount of population needed should dependend on who controls the city. or maybe you can only build each factions houses in their cities. otherwise this looks quite unbalanced.