Those Skaen cultists you're killing are human beings, they have rights mind you, and one of them is to not be in a bestiary.
More seriously, RPG usually wont have bestiary entries on "intelligent races", since they can be of any class, any level, with different items, so they're stats are supposed to vary a lot. And since they're not in the bestiary, you won't get any xp for killing them in PoE. Ever. It's not a bug.
