Achievement issues when owning both MOTN and MOTN: Remastered (Steam)

[Slaav]

I played the original MOTN before switching to MOTN: Remastered, and I can't unlock the Steam achievements I've already unlocked in the original game.

Like, I cannot unlock achievements such as Hisomu's Heir or Awakened, even though they're trivial and I got them in the original. But I can unlock achievements I didn't get in the original : I just unlocked Tactical Espionage Action, Inner Heaven and What Could Have Been, for example.

I was trying to 100% MOTN:R but I won't be able to unless I can fix this. I've tried verifying the files. I've just recently come back to the game and reinstalled it but I remember noticing the problem back when I first got MOTN:R, so I doubt it's a problem with the install.

[Slaav]

I solved the issue - kinda. The issue came from my savefile which I had imported from the original MOTN.

First I did a backup of my save (see this guide to find the file), then I "deleted" my active save in the in-game settings, then I played the first mission to see if I could unlock some achievements (like Awakened).

It worked. I ended up replaying a good chunk of the game on this new save in order to unlock all the achievements I couldn't get due to the bug, which took a couple hours, but fortunately the bugged achievements weren't too hard/time-consuming so it was a bit shorter than I expected.

Once I was done I ditched the new save and manually put back my old savefile, then went on to unlock the last achievements I needed. I was finally able to 100% the game yesterday.