I've started seeing this since installing Rise and Fall; two neutrally disposed AIs, often across the map from me and sometimes each other, would simultaneously declare war on me (probably joint war? it's a formal war but neither denounces me beforehand) out of the blue. Then they would proceed to not do anything actually warlike (other than maybe sending some errant unit like a caravel that was exploring nearby). After a while some declare peace. Some insist on having reparations for peace until I send a couple units to kill some of theirs... otherwise they refuse to settle (without me paying money) so this no-contact war can continue forever. What gives? I've never seen this before and haven't seen any reports of this but for me it happens reliably 3+ times in a long game.