TLDR first: The latest patch has fixed the problem.
Tough Nut unlocks now simply on Iron Mutant. The difficulty you're playing on doesn't need to be Very Hard anymore.
!!!This is a warning not a guide!!!
I just finished the DLC on Iron Mutant Normal mode and didn't get the achievement for it.
I found a user in the Steam forums of the game having the same issue. Another user stated that he got it playing on Iron Mutant Very Hard.
If you're going for the achievement you have to play on Iron Mutant Very Hard, it seems.
I contacted Funcom via their support forum on the 9th of August but haven't heared from them yet.
I will update the solution when they get back to me.
Edit 2: They finally got back to me on the 12th of August unfortunately only asking more specifics on how I started the DLC (Completely fresh? From an old savefile? etc.)
Will update when I get more.
You have to play the DLC on IRON MUTANT on Very HARD to unlock the TOUGH NUT achievement.
I got another response from them:
We appreciate your constructive feedback and will try to improve our communication in the future.
To further clarify on this situation, the issue had been registered and was being looked into as the achievement should unlock no matter the difficulty, as the description doesn’t specify difficulty as a requirement.
We’ve recently received confirmation that it will be addressed in an upcoming patch to reflect the above, and we apologize for the frustration the issue has brought you, as well as the fact that we didn’t get hold of this information within a reasonable time.
In any case, congratulations on earning it on Very Hard, we’re happy to know that you’ve enjoyed the game and we truly appreciate your support!
It seems it's gonna be patched some day.
But seeing how long it took them to fix Zone Master this will surely take some time.
Edit 5 (14th September 2019):
finally another answer:
We’ve recently got confirmation that the issue will be address for the consoles in the next hot fix ( together with other several remaining issues ), although we’re unable to offer any ETA as it’s still undergoing testing.
This sounds like it's gonna be the next thing that hits the console versions. I keep my fingers crossed for you guys and will update my solution as soon as I get more information.
Hotfix (18.09.2019): Hello Stalkers! We’ve just released a small Hot Fix update for the XBOX version of MYZ: You may find the notes below: Changelog (1.14): Bug Fixes “Tough Nut” Achievement no longer requires hard difficulty Several bug fixes
They finally fixed it. I asked for confirmation if you need to beat the final boss again or if the achievement unlocks retroactively.
Hopefully their last response:
Happy to help! You’ll likely have to beat the boss for it to trigger.
The final boss in the base game on Iron Mutant had a save right before the last turn to kill it. I hope the DLC boss is the same. Otherwise this would mean a completely full run on Iron Mutant normal to unlock this achievement.
If anyone could let me know that would be great.
Credit to MALFUNCTION II for being the fist one to confirm that the achievement now indeed unlocks on Iron Mutan on normal/any difficulty.
He did a full run starting with pre-set characters skipping mostly anything bedsides the main missions.