Unfortunately, the user who reported 102% completion said he doesn't know how it happened. Good luck finding a volunteer for the save files test.
That's what I gathered.

It took a few hours, but the achievements I was testing were recorded on the site as unlocked for Deus Ex Revision. It took a longer delay but eventually GOG Galaxy showed them as unlocked as well.

Now the issues you listed remain, so I'm going to shelve the game for now. This thing has a whopping 255 achievements and most seem fairly difficult. I don't want to spend ages reaching that 100% milestone only for progress to be wiped if they decide to fix this tangled mess.

I opened support ticket 1151556 yesterday. My support response time has been in the 2-5 weeks range, so it'll be a while before I have anything to report back. How about you? Did you try contacting support about this at some point?

Yes, it's in the "Achievements working with errors in titles or icons" list in post 6, and I posted in your subforum thread.
As Hustlefan says NISA hasn't been very cooperative, but I've heard that Durante handles these matters, so if you know how to contact him directly, perhaps via social, maybe that's the easiest way to get something done.

I already tried to contact GOG support when the only problem was the missing icons, and they replied with the usual vague promises. In fact they have corrected the icon problem by creating this new abomination. I won't be opening any more tickets for now since I already have other things going on, but let me know how it goes.

If you have Discord you could pop into our server for more agile conversations and the latest news on Galaxy achievements. The invitation is open to everyone of course!

I mentioned this elsewhere but I noticed Bear With Me is still on the list.

Is the True Detective still broken? I thought it was already fixed.

Some games can be completed beyond 100% but I actually don't mind testing that. We got copies of Mad Max about the same time with Ice_Mage but I have not managed to play it yet.

Just give me a moment. Next week is Far Cry 6 release and I am going to be playing that (with one of the most intrusive DRMs ever -- Ubisoft's. And ignoring all the microtransactions which I am sure will be plentiful). But after FC6 I can play Mad Max.

Is there anything I should keep in mind while playing it? Which achievement is that problem one?

UnMetal achievements have been fixed with the latest update that just got uploaded, so this post is no longer relevant.

Bear With Me: Yeah, since the last time we talked about it, GOG was supposed to fix the "True Detective" achievement, which if you remember wasn't unlocked automatically. After that there was the summer vacations and I haven't actually heard anything more from the developers or GOG, so at the moment I'm still not sure if "True Detective" works. It would take someone to try to complete the achievements from the beginning, so for now I would leave it on the list as a precaution.

Mad Max: The achievements you now see on GOG should all work. The question remains whether the achievement "Up to the Task", which isn't visible at the moment, remains latent and whether it's possible to unlock it, exceeding in fact 100% completion. The only way to unlock it on GOG was to use an old save game, like this user's save: https://steamcommunity.com/app/234140/discussions/0/2592234299562834210. So if you want to try the experiment, first complete the game normally, and then try using the save game in question and see if it unlocks the achievement.

Oh, right. Now I remember. And yeah, I can't really test that so it would require a volunteer to test. (But it's a fun game so basically... anyone who starts playing it.)

Oh, ok. I will test that after FC6. I wasn't planning on 100%ing Mad Max, but might as well. Already downloaded the save file for later use.

I'm using the old version, Galaxy 1.2. Although I doubt that would make a difference to this problem.

Also, the hint given in the OP for this game does not resolve the issue in my case. It doesn't matter how many times I restart the game: no Galaxy Achievements ever unlock for it on my account.
Indeed someone should contact Durante about this. And also it would be good if he could finally fix the Achievements in Trails of Cold Steel 2, which have been broken for years with the latest patches. Durante is also the one who ported that game and did the Achievements for it.

So if anyone does contact him, then please ask him to fix Cold Steel 2's Galaxy Achievements as well.

Just follow the link frogthroat posted. It actually has a screenshot attached to it which proves that the achievement is working again.

Unfortunately I don't have the game to test it, it could actually be that an update broke the achievements. It's worth trying to send a ticket to Klei support at this address: https://support.klei.com/hc/en-us/requests/new
Let me know how it goes and what they say!

That's right, Durante would have the authority to solve these problems but to put it mildly he doesn't seem to care much. I had tried to contact him but he never got back to me. If anyone else wants to try, this is his Twitter profile: https://twitter.com/Durante_PT

The GOG staff unlocked it for frogthroat because there was a complex bug with duplicate achievements, so now we don't know if you can unlock it normally unfortunately.

Ah okay.

In Flynn: Son of Crimson, the achievement 'Hey There, Bax!' - find and speak to a certain character in 4 locations - isn't unobtainable, but this task needs to be done in a single play session, so no quitting to the main menu or anything like that. It isn't difficult or time-consuming, but I don't think the behavior of this achievement in this way is intended.
Edit: This also affects the game on other platforms, it's not just a GOG thing.

Recently started playing Of Orcs And Men and achievements were going fine until today. Trying three times to get "Well Trained" and "Hand Shake" in Chapter 1, no luck.

(I had no idea achievements were such a problem with GOG Galaxy until seeing this thread. Might have to think twice about buying certain games on here where I might like to try for 100% completion... very disappointing)

Gog should implement the same achievement system as epic, so there would be real support. In addition, Epic provides the tools for it.