I reported it and GOG responded:
"We are aware of the problems related to the achievements endpoints not reflecting changes introduced by the client in the way they should. An appropriate department is already looking into this issue and we will do our best to fix this issue as soon as possible. There is nothing that needs to be done from your end. Once the matter is addressed, everything should update itself automatically and the percentage of unlocked achievements will be properly adjusted the next time GOG GALAXY client is restarted."
Perfect, i ticked today but still waiting for the answer, so we just have to wait i guess.
It's not promising reading post from 2020 with people with the same issue, and we are here 3 years later with the same problem, but let's see.
It has already been fixed.
Things should be back to normal.
