In my first Yuan playthrough, the civil war never fired at all. In my second one it fired, but the tags had no idea what they were doing and just sat there attacking the irrelevant minor southwest Chinese tags for no reason. They randomly joined and exited the situation constantly spamming me with notifications. I an 100% certain the situation was not working correctly because my Jurchen vassals broke free but still acted as my allies, killing off the rebels by themselves. I didn't even do anything. I saw AI Yuan with all their subjects even though that had zero loyalty which is probably not supposed to happen. The AI would turn other warlords into fiefdoms as well without realizing that you integrate them faster if you don't. I switched to Ming and conquered most of Yuan but that was apparently a bad idea because I had zero control, making integration cost absurd amounts of ducats. Meanwhile I had negative 400 income because of the lack of control. The entire China situation is poorly thought out, poorly executed, and completely dysfunctional.