The damage of self-poison that Aura of decay applies increases with the level of the area you are in. On my character (lvl 66) it ticks for 19-23 self dmg in lvl 5 zones, 53-63 dmg in the end of time and in the Arena at wave 80 it ticked for over 100 dmg.
I assume that you are considered an enemy for Aura of decay’s poison penetration calculation.
My oppinion on this scaling:
It’s an unfun and unintuitive mechanic. You will reach a point where you can finally sustain the aura and then you progress through the campain or the early endgame to higher level zones and suddenly you start degening again and have to go back to lower zones to craft more regen even if you outgear the enemies already. If you don’t want people to be able to permanently sustain the aura you should rather add more self damage on the damage nodes of its skill tree.