I dragged two such parties into a fight and necrosavants drains hitpoints from them. I cant say if anaotmist potion does the same or not for my save is away from trying that. But, doing that would be a bug; and if I cant drain hitpoints, necrosavant drainig hp should be a bug. This happened sometiem ago so ıdk how to trace old combat logs of if they are usefull at all.