I was playing as the Gongsun and was given the mission to form an alliance with wu and shu. Wei had just abdicated their throne to me. Wu and Shu were already in an empire, so I joined it. I believe that Shu was the head of the empire but when talking to him I don't have the option to leave it. The mission was not completed by doing this.
When feuding with him I get the option to annex his settlements, and he can do the same to mine. However now my game frequently crashes almost every time I try to progress.
Is being unable to leave an empire a bug? Is being able to simultaneously annex and be annexed a bug? I'm almost certain the crashing isn't a feature.