Worry not comrade P3, if you are a dedicated achievement hunter, i shall try to help you !

First things first :
Make sure that you
a) completed the whole main game and did talk to 50(prolly less cus buggy corpses) corpses. You should be stuck at around 45/46 corpses.

b) Make sure you tried the "Talk to 3 animals, save, reload save".

c) reinstalled the whole game (I did it before pulling my hair out, probably didn't have to)

d) verified files

e) prayed to machine gods.

Okay so if you are stuck like me, what worked for me was that while i was playing my third game (ng+) i decided to talk to the "Open world corpses" again because every time i talked to them, the achievement counter popped up but nothing happened. I don't know if you need to do this in a new game plus, or if you can do it in a new game only.

What i did was that i talked to every corpse in New game +, that's when my achievement counter went from 43 to 45, then i almost finished the whole game, reloaded a save where i was last in the open world and decided to talk to the open world corpses. I talked to (in order) the dead chicken, dead bureaucrat, dead milk woman, stinky corpse, dead farmer.

I have 0 clue if the order is important, or what i did is important, i just wrote it down for anyone who is struggling, maybe someone smarter will figure it out.

Corpses that trigger the counter but did not change it for me since 43/46 :
Chatty corpse
Chatty corpse
Dead florist
Fallen corpse
Strange corpse
Dead weird scientist
Dead farmer
Stinky corspe

Hope this helps someone !