Well what kind of poison? Some poisons are absorptions just made into a potion and applied to a weapon. If so absorption spells should cause a reflect considering it's magic? Just my theory. :flamethrower:
I would imagine that all poison effects work the same way, only effected by weakness to poison, and resist poison, I see no reason why specific poison effects would be exempt to this, though I might not be an expert on the subject, but I think that by now, someone would probably have determined the details of how spell reflect or absorbsion work, and would know if they effect poisons.
If enemies are not poisoning you, I'd say that they probably either did not use poisons, did not get a chance to hit you after poisoning their weapons, or you have resistence to poison.