It's worth mentioning that this sort of thing can happen when the corpse is looted too quickly after the creature is killed. The Chieftan always spawns with the oil, but it has to get moved from his inventory to the loot bag before the bag is looted. If it does not, neither loot nor quest items will appear in the bag. Normally, this happens very quickly, but lag can delay this for a few extra seconds.
Quest treasures should be spawning better now but still no guarantees as for what Dorg mentioned.
Worked fine for us last night! :)