1. Hardened is easy...
2. You likely hit the "lower difficulty" option while trying to manually reload a checkpoint. This has happened multiple times to me during my playthrough, prompting me to restart the level from the beginning.
The game shouldn't take you longer than 6~ hours on hardened, my vet run took me just under 9 hours, on both titles. Just redo the levels you're missing.

Originally posted by It's about sending a message:

1. Hardened is easy...
2. You likely hit the "lower difficulty" option while trying to manually reload a checkpoint. This has happened multiple times to me during my playthrough, prompting me to restart the level from the beginning.
The game shouldn't take you longer than 6~ hours on hardened, my vet run took me just under 9 hours, on both titles. Just redo the levels you're missing.

Except I didn't. I redid Crew Expendable on Veteran, then save and quit - and it still shows as Regular.

I then played without the Cluster Grenade cheat because I thought that was what was causing it to not work, but again - still shows as only being completed on Regular.

Originally posted by judgefidler:

Except I didn't. I redid Crew Expendable on Veteran, then save and quit - and it still shows as Regular.

I then played without the Cluster Grenade cheat because I thought that was what was causing it to not work, but again - still shows as only being completed on Regular.

Well I don't know what to tell you. Worked just fine for me when I played through it. It also goes without saying you should've disabled all the cheat unlocks from the get-go, as cheats usually disable progression like that.
Make sure you have actually everything disabled and then start a new game and see if the difficulty sticks. Alternatively delete/move your save and create a new one.