Conquered city returns to vassal?

[denogginizer]

Not sure if this is a bug or not, but I find it very odd...

I was at war with Hammurabi and successfully conquered all of his cities on the main continent, though he had a handful of cities on various islands. I opened a dialogue with him, to which he agreed to capitulation.

Several turns later, the Celts declared war on me an conquered the last city I took from the Babylonians. Once I moved my army to that side of the continent I took the city back. However the culture indicated it was in Babylonian control (who is still my vassal) instead of my control.

I left the city empty (no defense) and let the Celts take it back. I moved my forces back in, and this time when I conquered it it was placed in my control.

 

[Virulent]

Something similar happened to me. I was playing Babylon and I was in war with the Aztecs, I took several of their cities and vassalized Monty. A couple of turns later one of the cities I had taken from the Aztecs culture flipped to nearby Portugal. Later on when I went to war with Portugal when I took the city back it was instantly returned to Monty.