All pacts do last 50 turns. This includes the non-aggression pact. I am not sure if you can check for how many more turns will it last, however by itself, this situation is not a bug.
Backstabbing mechanics are not in the game, and not many players would like those. It is a valid request, to have such a thing optional in game settings, though. However, as far as your playthrough goes, you will have to stick to the pacts signed with the AI.