The enemy/AI had 3 cities with overlapping borders.  After I conquered 1 city, borders/ZoC around the remaining cities became corrupted.

Save before conquering city

debug.err for this session

Here are the borders before conquering the first city:

• Blue = Pariden (me)
• Red = Altar

Here are the borders after conquering Nikost.  The borders aren't corrupted yet, but Altar still shows a north border (red arrow) that doesn't appear to have updated yet (Altar has no cities to the north/east now that it lost Nikost):

After hitting End Turn once, the borders around the 2 unconquered cities become corrupted (green arrows).  The northern border appears to have been corrected (?):

Same borders in 3D (easier to see):