That sounds strange.
From reloading the game, it seems to have ended after your opponent bought a City. Do you remember the Mountebank being played, to which you discarded a Curse (might have been autoplay)?
So the game considered you to be the one makeing the next decision, so it likely timed you out and your opponent made you resign. Then the question remains why on your side it looked like your opponent was still on their turn. Do you remember an undo was happening that turn or you had to reload your browser tab?