Vassal's occupied city bugged? (Humankind)

Started a war with an AI that had a vassal. I captured the city of the vassal, since the vassal automatically fought against me like the overlord. While I was at war with the AI, the overlord let the vassal go. The city of the vassal remained occupied by me. After the AI ​​gave up, I did not find it in the list of requirements for this city. And the city remained occupied. I cannot loot the city or any cell, since it is occupied. And no one wants to attack this city. The people of this city are gone. How can you get out of this situation? Or is it a bug?