Whats up with Rath's genetic siphon?

In the story Rath’s genetic siphon doesn’t work on Guardian type enemies and Rendain himself. This makes the Sentinel mission and the Heliophage very difficult for Rath players. Is there a reason for this? Can a Sustained not be sustained by another Sustained? (lol) Is it something lore wise that doesn’t let you gain life from non-organic life-forms? Which if the case why can i gain health from robots? Or just a bug? I need answers!

Have died many times trying to get health. Never sure if its working because the amount of hp gained is so small.