I've been testing the void worm fight and I've noticed that if you manage to split the worm into pieces, its death is no longer registered correctly. To be more specific, I the first time I fought him he split and I didn't get the achievement and almost no XP. Strangely, the mandible and eye did still drop as intended. However, when I fought him and managed to take out only the head, I finally got the achievement and a big chunk of XP. I believe this is unintended.