Quoting Raiddinn, reply 6
Try renaming the town to something else instead.
Have you seen this work?
I have never seen the bug.
I rename all fortresses to "Fortress", all towns to "Town", and.. you get the idea.
That applies to all cities that I found and that I conquer.
I am willing to bet that some of those towns probably had the right name to have it get bugged and that renaming it fixed the problem before it became one.
That being said, I definitely can't speak for sure on that.