Black Knight Challenge misleading pop-ups

This is the second time where I find somebody to carry my Black Knight fight to be sure to get it, I sit with my spawn protection, they kill it, I see the pop-ups on-screen of the challenge being completed, but when I actually check in the list of my challenges its not completed.

It's driving me crazy because one time I can understand it being a random bug, but two times? What is going on? And if I have been failing it, why do I see the pop-ups of the challenge being completed?