Zevkk

It's definitely bugged. If you kill the witch when she's in animal form, she doesn't drop the grimoire. I think there's also an issue with the Lich, but I don't know what it is exactly, since I was able to obtain his. Other people have reported it as an issue.

Apparently, there is a bug that causes both the Lich and the Witch (Bipara) not to drop the item. For the witch, it's associated with killing her when she's in animal form. No way to get them without console commands if you don't have an earlier save, unfortunately. I can't complete it either due to the witch bug.