I have seen this twice now just recently, and do not remember similar behavior in the past, but maybe hadn't really run across the circumstance.
When you are at war with some other faction or factions and the UP resolution for Galactic Peace passes, it does not take effect right away. In fact, it does not take effect until the the start of the next turn. This gives everybody involved one last chance to get a parting shot in and do whatever damage possible before they absolutely positively have to stop.
The diplomacy screen will still show the war status as active via the big red lines. You can attack things without getting a "this will start something" warning. You can snipe constructors, starbases, shipyards, and asteroid mines. I know because I just watched the AI pummel my infrastructure one more time in a vicious move that would have shattered any real life treaty big time. They even invaded and took a planet! And then suddenly we are all at peace. I admired the creative evilness of it, was maybe even jealous, but I suspect it is not what is actually intended to happen.
I have seen this in both MP and SP. My SP is heavily modded, but the MP is definitely not. So, my questions are: Has anyone else seen this? Is it a new behavior? Is this WAD? Is it sporting to get one last jab in as the ref breaks you apart?