MitigateMeleeDamage Never Fading?

[Hateborne]

Is there any known bug/problem with partial melee runes? I have an ability essentially copied from Panoply of Vie (mitigate 10% until 2080) that has Mitigate 15% until 50,000.

It never fades...

I let it run for ~20min, tanking several mobs, and it never faded. I kept the 15% melee mitigation and took ~400k damage but the ability never faded.

Now the best part is Panoply of Vie WILL fade, this copied spell with raised mana cost, mitigation percentage, and rune amount never does.

Any pointers? :-\

-Hate