Invisible and invulnerable City-state

[Logan of Arabia]

I moved a unit onto a city state i was in the process of befriending to protect it from being dispersed(I did not disperse it myself). After about several turns the city-state mysteriously vanished, but the territory of the city-state remained locked and in the name of the city state. The AI is still unsuccessfully trying to merc the ghost city-state. Its very funny.

[TheNightglow]

is there a resource below the city state?
there is a bug where city states will just glitch out as soon as they are befriended if they happened to have spawned on the same tile as a resource

[Cruise the Cloud]

try to save game and come out to main menu and then load save back in to the game some time it's help to fix those glitch if not just bare with it until next age...