It seems that Overkill introduced a glitch a few days ago that means the achievements for doing all of a person's heists (and all heists in general) on a particular difficulty level are unlocking prematurely. Thought I'd just post a link to the official confirmation of the glitch just in case people think the affected people had been cheating.
http://steamcommunity.com/app/218620/discussions/14/35054427...Of course, if anyone wants to take advantage of this... well...