As head of the UP, I had selected Galactic Peace to vote on, and the vote succeeded. So even though there was a forced stop to hostilities (I was at war with the Yor), a Yor ship still managed to attack and destroy a starbase of mine. I wasn't given the option to retaliate because of the forced peace.
I'm guessing the bug here is that the Yor ship was already assigned to attack the base as a path, and the AI was never told this was disallowed because of the peace. I believe the attack happened a round or two after the vote.