A couple of days ago, on patch 1.0.11, I ran into a strange situation during a game where a Lumeris AI that I was at peace with sent me a message indicating that she was going to attack me. The next turn, she was able to propose a truce, despite not having actually declared war; we were at peace the entire time. You can see this in my Let's Play here: 

Turn 92, when the threatening message is sent: https://youtu.be/fdxP5gLSbmo?t=2312

Turn 93, when the truce is offered: https://youtu.be/fdxP5gLSbmo?t=2502

Frogsquadron asked in the comments if I could send you guys a save from around the time that this happened, but unfortunately my autosaves don't go back that far. In addition, by the time I had seen his comment patch 1.0.18 had been pushed, and I don't seem to be able to replicate the issue now; I can get the Lumeris player to send the threatening message on turn 92, but not the truce offer afterward. However, after sending me the threat the Lumeris player's diplomatic modifiers are still pretty weird (they contain both "We are winning the war" and "The war is balanced again", despite the fact that we are definitely still at peace), so hopefully you guys can still get some idea of what's going wrong.

Possibly relevant saves:

The save from turn 77, at the beginning of the linked LP episode (made during patch 1.0.11): LP Riftborn 3.zip

The save from turn 102, the end of the linked LP episode, after the weird truce has been accepted (made during patch 1.0.11): LP Riftborn 4.zip

A save from me trying to replicate the conditions this morning, on turn 92 right when the Lumeris player sends me the threat (made during patch 1.0.18): Riftborn 92 Aggressive.zip

I was at war with this Lumeris player earlier in the game, so maybe that has something to do with it? I hope this is enough information for you guys to track down whatever's going on, and let me know if you need anything else!