So, I completed the game on Death March with level upscaling and only received one achievement for finishing the game in any difficulty, so wtf happened. I finished the game after I finished Blood and wine and Hearts of Stone. I tried to play Something ends, something begins again but yet nothing happened. It's really annoying since I was triying to get every achievement.

You need to start a new game and finish it in death march diff and dont change it at any stage.
You propably did that but i will just put this info out.