I thought this was a feature :P lol although I have only seen it happen on the campaign during .8xx or. 9xx . I have not noticed since 1.0.
chopki
Crap didn't know Krax had a spell that good! it was Krax who stole the city so that seems possible. The bug for AI win was definitely conquer, had a cut scene pop up and all stating a conquer win - it was Krax who won so maybe the AI managed to cast the spell on all of my cities in one turn, and then when I re-loaded they decided not to? - I don't think I had difficulty that high, maybe one up on normal. (ill check later)
Have just played about 5 hours into a game and the AI have managed to win via conquering without taking any of my cities and then also an ai declared war on me and managed to take one of my cities (that had units and defences) without me being notified at all! I have seen minor bugs before that have been annoying but not like this! - managed to get past loosing be being conquered by loading a save game and replaying on.