I've seen this a couple of times, most recently yesterday. I ask an AI to join me in declaring war, and they say the need 10 turns to prepare. So I declare war without them. The war goes well, and I capture all the cities I want and thoroughly pillage whatever is left. Then I offer a peace treaty. The next turn after I've made peace, that AI that I asked about the joint war pops up and says it's time, and I decline (I don't know if I even can accept because of the peace treaty) I never agreed to wait 10 turns. For the rest of the game, I have a bright red diplo modifier that I broke a joint war promise. If they had asked me to goto war and I said yes and then backed out, I have no problem with that. But I initiated this and I made no promise.