How are Blacksmith Whetstones supposed to work?

I just bought and regaled a bow up to "90% Increased Physical damage" part it was a hybrid. Them I spammed Blacksmith Whetstones on it to 20% Quality.

But, now the "90% Increased Physical damage" is gone. It says "21% Increased Physical damage" instead. Btw, I can still see the accuracy part of the hybrid mod.


Do I do something wrong, is this a bug visual or hard broken?