I was at war with "green", who at the same time was at war with "purple" (0 fights between them after 50 turns). I crush all armies from "green" and occupy it's only city. Next turn, "green" surrenders to "purple" and becomes it's subject before I can seal any peace deal with "green".

It makes abosulty 0 sense. There should be a certain "war score" like in "Europa Universalis" or any other Paradox Game, where actions during the war counted for something, aside from "war support".