New city for me?

[Yzman]

Ok so I am playing a game right now where I am Portugal and currently I have 3 vassals: Zulu,Carthage and Egyptians. They are all voluntary. I was at war with the Germans when all of a sudden I saw a message that I captured a city...when I clearly had not done so. I looked and found the city, it was a tundra city. It wasn't on anyone's land but was closest to me. Outside the city I saw a zulu unit. I had no units inside the city(it was undefended).

How did I gain this city?

 

[oranges]

Presumably, one of the bugs introduced by 3.13 patch and fixed by unofficial Bhruic's patch.