well, I suddenly have achievements for both DLCs in the list, they don't unlock however, at least not for the first dlc which I finished while the achievements were missing. I loaded my save, finished it again, no achievement, tried 2 other achievements, no unlocking.

Anybody gotten DLC achievements yet ?

Nobody's posted about it to https://www.gog.com/forum/general/list_of_broken_gog_achievements but maybe that just means you get the honour. (Personally I'm too early in the game to check.)

Same here. Achievements for DLCs are broken.

I have no problems getting the DLC achievements from both DLCs except for the secret Postman achievement when I sent the Journal to Lucien and it didn't activate until when I exit the game and start the game again is when the achievement decided to unlock which was weird.

So maybe before turning the quest in, saved, then turn the quest in. If the achievement doesn't unlock, then quit the game and start it over again and try it again. If that doesn't work, then I don't know anything.