When demanding peace from a custom created Sovereign I keep having issues where they agree to become my vassal and then vanish entirely from the face of the map never to be seen again.
When attempting the Quest for Mastery I refused the dragon's challenged and said I'd come back later. The window that first initiates when you bring them both dragon eyes then proceeded to pop up every single turn I took no matter where on the map I was. This also resulted in my Sovereign, (and any army they were in) to be teleported by "Call the Titans".