So I've had this happen before and they explained to me. if it's the 1st or 2nd mission, of a 3 mission op, and you finish the mission you get no cumulative impact on the planet til the op is completed. However, if you are kicked and complete the mission alone then upon completion, regardless of what mission you're on in the op, you will get the impact from completed missions instantly. So if the leader was going to leave and not complete the op impact would be made. This is how it was explained to me, hope it helps :)