I heard mention that this was something that was fixed a while ago, but I just noticed a huge difference in self poison damage when I was spamming soul feast with this new character.
Soul Feast’s self inflicted poison at 5 stacks ticked for 18 at the max with 30% poison protection.
Aura of Decay, with 55% reduced poison damage taken coming from the skill itself, along with 37% poison protection (after it ate through my ward and leveled out degen/regen) was ticking for more damage at 3 stacks of self poison.