On Xbox One, when selecting a mission you have an option to resume by pressing "A" or restart by pressing "X". I tried that today and apparently it didn't work. I had the bounty for killing Decisive Mind and it would not allow me to restart. It kept putting me back where I last left off... after that point.
-
Okay, I found out that you must press "X" when selecting the mission. Then press "X" again when selecting the difficulty level.