Its rare to happen on army vs army, but on sieges it was 100% of the cases for me. I cant finish it because they quit as soon as they notice they will loose. I would say its broken since is pointless try to siege someone: he will probably quit just to not give his city or the victory to you. I think if player log off we should win or we could keep palying and his army would be controlled by AI or not controlled at all ... If that happens I doubt someone will quit.
But right now, well, I couldnt finish any siege that I have started. It has become a major issue for me