It's now 2022, I just recently started playing D:OS EE (I bought the Eternal Edition) and I realized that I don't seem to be registering achievement unlocks.

I'm on macOS Monterey 12.4, GOG Galaxy Version 2.0.54 (22), and D:OS EE 2.0.119.430.

I'm playing as single player, exploration mode, if that makes any difference. I know that some games only register achievements of standard difficulty or higher, but don't know if this is one of those.

I'm not a Mac user and unfortunately I can't help you but I don't think that the problem is caused by the difficulty level you chose. As a matter of fact, according to this post, you are not the only one who experienced this issue.

Yeah, I had seen several posts about achievements not registering, but most of them were a couple of years old, so I thought I'd ask around to see if anyone had any updates here. Alas, it would seem that there hasn't been any change as far as we know, but I did send an email off to Larian Support with a couple of thins to try, which I'll be trying this evening.