Hmm! There was no perm alliance between them, but now I'm remembering some sort of other bug the other day I spotted. Chariots don't get defense in trees, but I noticed, if a friendly neighbour of mine happens to be stacked in the same tile as the chariot I want to hit, this fubbers up the whole combat odds calc, and then it applies the +50 forest def to the charriot. I'm not 100% sure if it's just a display glitch or if it actually will affect the combat though. Since that happens, I wouldn't be surprised if Qin maybe had a unit in the city and caused this to happen. I do remember, pre 3.13 when if there was a friendly spy unit even next to a city, you couldn't nuke it at all. Anyhow, I'll keep my eye out for any more anomalies.