Sounds odd.  Do you have screenshots to share?

Yes:

i

i

Are  you  still  seeing  this  issue  and  if  so have you tried restarting your game?  

Game restarted many times, now the CB is reset :/

I opened a bug report.

Sounds good! Let us know how it goes. :)

Still no response from Plarium :/

You need to deal 100M damage After you get the mission. Any damage dealt before you get the mission will not count. At least that is what I remember. 

I had a problem with this cb missions on arbiter as well, I made a post about it. Can't remember but I think my nightmare damage wouldn't count but my unm did or the other way around, the quest was def bugged for me but I got it done from getting credit from the wrong version of the boss at the time.

All damages made on NM CB and after getting the mission.

Only the last key was not taken in account.

It'll likely be Monday, their offices are in the Eastern European Summer timezone, so they basically head out on Friday as I'm getting up Friday morning. :)
 

If  you  receive an  automated  response  make  sure  to  reply  to  it.  

Plarium's reply :

"We have thoroughly tested this element of the game and found no bugs. "

😈

No comment.