Trails in the Sky SC: Master Fisher is hopefully fixed now.

https://www.gog.com/forum/the_legend_of_heroes_trails_in_the_sky_series/is_the_master_fisher_achievement_for_trails_sc_bugged/post3

Wonderful news! We should take the opportunity to mention the other bugs, maybe it's the right time that they are fixed.

Serious Sam Siberian Mayhem

Achievement doesn't unlock after completing chapter 2

Unfortunately, it seems that the Renowned achievement from Banners of Ruin is broken. And it appears that nobody on GOG has been able to unlock it.

DOOM BFG Edition:

EPISODE 1 and EPISODE 2 achievements do not pop. Tried beating last levels of those episodes a few times. Still no pop.

Edit: Tired on Hurt Me Plenty and Ultra Violence difficulties.

Do you know what the achievement is called exactly? As far as I can tell, there are two achievements that it seems no one has ever managed to unlock. You could try to contact the developers, when I contacted them about an achievement problem in The Talos Principle they fixed it very quickly.

Yes, this one is definitely broken, if you've contacted the developers let me know if there's any news, for now I've added it to the list, thanks.

This is a strange case, I know many people who have completed them and the game hasn't received any updates, I would suggest you try again with a new save to make sure there are no other factors at play.

Perhaps you have any tips on what could be the problem?

Ok, I have played this game on steam before and unlocked said achievements there. Any ideas how to fix this Steam problem so I can get them on Galaxy?

You should try uninstalling the game, both GOG and Steam versions in case it's still installed, and make sure there are no saves left on your computer. And then try again from the beginning.

Thanks man, I owe you.

Update to my support ticket about this, opened February 15th:
• First reply received April 5th. I was only asked if the issue persists. Yes, it does, for the games I'd already mentioned in my follow-up replies: Blackwell Bundle and The Uncertain: Light At The End. The latter doesn't have achievements newly added. It's also missing the "Visit store page" link in the achievements list view.
• Second and final reply received today, April 11th. Ticket closed with, "our team will work on proper solution in the near future". No estimate of when that'll be, as usual.

I had no expectations and I was still disappointed. Back when my average support response time was 5 weeks, I didn't imagine support would get even worse.

Thank you for the update. Unfortunately I agree that support is now intolerably slow, however I have managed to get some results with a ticket I opened on December 13 and continue to receive responses veeeeeery slowly.
I have been able to fix invisible achievements issues or have completely broken achievements removed, and fixing the issue you cite is the next goal.

If you noticed, the problem persists when the game in question is sold on GOG as part of a package, in that case it's the package that has the feature and not the game itself, creating the bug on profiles. The only thing GOG has to do is to put the feature in the specific id of the game itself and it would solve the problem. But I want to be hopeful that this, too, should be resolved at some point.

Good grief. I thought having an open ticket from January 1st was bad. Though I've previously had a ticket drag on for what must've been over 6 months, and in the end the support agent stopped replying and silently closed it. So it can always be worse, it seems.
Thank you for your efforts.
It's weird that The Uncertain: Light At The End is a package, since it's just a game with no additional content. But sure enough, that's what GOGDB shows.

Not sure if this is a GOG problem or a game problem but for the game This War of Mine, Im having trouble getting the Malik on Duty achievement. You apparently need to listen and broadcast all 26 comments which can be obtained by day 20 which ive done serveral times but the achievement doesnt unlock.

This could be a game problem since steam users also have trouble getting this achievement.