I started on supersoldier and got the 6 & 12 missions achievements for supersoldier. finished the game and did not change level of difficulty off of supersoldier and did not get the completion achievement. note you also get veteran achievements.

Cool story bro, are you looking for a prize, a cookie, or a sticker?

Weird I got all of my Campaign achievements and my completion.

It's a known bug and i think the only way around it atm is delete your save and restart