Hey, I recently finished the campaign but did not get the "Finished Campaign on X" achievements.
I played all the missions on Hard, changed the difficulty setting to Normal for mission 09 and 10 (The Seer, the Trickster and the Brute; Cunning and Wiles respectively) and then switched back to Hard.
I then went back and re-did those 2 missions on Hard, and did get the achievements for finish missions but not for finishing the campaign itself.
Is this a bug?
The thought crossed my mind that since there were a few "overlooked" gameplay assets (such as 'view campaign map') the Campaign isn't actually done with yet.
Furthermore, I was wondering if Re-Doing the entire campaign would let you grind more EXP again for Elites. In doing Cunning and Wiles on Hard I only got about 7000 EXP per Elite despite them inflicting huge amounts of damage, when originally on Normal it gave something in the range of 150k.