When the Expedition arrived, I immediately attempted to dissolve my alliance with the Reticulans. After I did it, I got the opportunity to attack their territories, but in Diplomacy they are still listed as Alliance and the Expedition refuses peace with me, I suspect because the game thinks I'm still allied. Is there a way to fix this or should I go cry over their official forums?

If i remember correctly you have to actually attack them to officiallt break the alliance, so go ahead and conquer one of thier tertories

Yup, so I've been told at their official forums, thanks Quite a rather senseless "feature" though, I'd call it a bug if it didn't seem so intentional...