This happened on my game as well. The other players were still able to place a bounty on me despite Truce Among Rogues, and when the pirates dutifully dispatched a fleet as per the bounty, I would get the pirate attack music, a fleet would arrive, I would get the "a friendly fleet has arrived" notification, and eventually nearly all my home system planets had giant pirate fleets sitting on the edge of the grav wells, doing nothing.
