Noticed something odd in my most recent game. The AI settlement of Queenshand (behind the arc of mountains) has a outpost underneath a settlement. The outpost was built first, then a few turns later a settlement was built on the same tile. The area of influence then decreased (since an outpost has 2 tiles influence and a city only starts with 1). The outpost can still be selected under the city.
I stupidly opened the game again so there's no debug, but here's the save file for it. The issue is with the city I mentioned above (Queenshand):