It seems odd that Bethesda added a completely useless poison ability like Damage Endurance.

Has anyone made a fix to make it work like it should?

I thought Realistic Fatigue fixed this. Damaging NPC fatigue will cause them to fall over and trip/stagger more often, plus pant when their fatigue gets low. At higher levels it doesn't show up as much tho, due to the incease in fatigue and regen etc