Twice-Fanged set (penetration stacking) #UPD

Ruinhorn · January 2017

I've tried to use this set and noticed that both OmniStats and Harven's Extended Stats show me only +1k of penetration without stacking.
Made several screenshots to show the base penetration and during the fight.

I'm a bit confused, because Alcast on Reddit told that TFS worked for him after testing.
I don't know how such addons work, so could it be just UI bug or the problem is real? Haven't tested numbers yet.
But seems I'm not the only one who noticed this.

#UPD
Just used it without any buffs and 1/2 Kragh set (to prevent its proc)
With 5/5 TFS the damage of LA was increasing up to 5 times (=stacking).
With 4/5 TFS the LA damage was stable.

So, it seems, that addons are getting wrong info without counting every stack of penetration and in reality this set works.