I dont know if its a bug, or if it has been overlooked.
When you conquer a kingdom city as an empire for instance, after the first turn the terrain flips to your style, but the actual city itself remains in complete kingdom design..
They should flip to your empire design as the terrain flips. I understand that kingdom-specific buildings such as huts or whatever wont flip (though they could, if you made them flip into the empire equivalent, kingdom workshop into empire workshop etc.)..
But at the very least, the city center and especially the outer walls should flip to your empire style (or vice versa if you play as kingdom and capture an empire city).
And possibly, if there's a Hedge wall built, it should be flipped to the empire Wooden Wall equivalent.
This would add much needed "roaaarrr i am converting the lands and cities into darkness" flair, beyond just the terrain texture.