Mortal Blow does not work

Description of the bug: (optional screenshot/video):
Mortal Blow should kill monsters at a 10% rate if they are below 50% hp.
Playtesters have shown that even with over 60 mortal blow hits, Imperial Guard As were not dying. The odds of this occurring with Mortal Blow completely functional is astronomically low.

What is supposed to happen?: Mobs below 50% hp should die to mortal blow 10% of the time.

Where did you find and/or notice this bug?: NTP2 on Imperial Guards, but should be checked on other high hp mobs like Jiaoceng as well.

A step-by-step guide to re-producing this bug (optional): Make Imperial Guard A hp lower than 50%. Use Mortal Blow with MM/BM until mob dies.

Extra information (optional):
There are a couple of different use cases for this. The most immediate is for Imperial Guards to farm the fragments. The other would be for Jiaoceng mobs that have 15m hp each. If mortal blow worked, archers would have an alternative strategy of mortal blowing 50% hp the extras to speed up their run.

 

Yep yep. Less than 1%, and it was repeated by multiple players back to back, 60 being the low count.

 

I hope this doesn't come across as an Archer buff. :^)