I was playing a long 10+ hours game on the biggest map. Me and 3 more AIs were on same team vs other 3 factions, not teamed.
Everything was good and fun until one of my allies on very strategical position resigned, 20- 30 minutes after him another ally resigned.
They were not the best score players, but not the worst either. One of my enemies resigned too after I used on one of his towns the earthquake spell. They had more towns left too.
Is this a bug? I hope it is, because I really love to play teamed with AI players and don't want them to abandon me. They should do it only when they lose their last town.