You may want to run through each level again, or check to see if the mission possibly asks for % stats on any of the pieces

Hi. It doesn't say that but I have done it.

I've gone through and got all artifacts with atk % twice now trying to complete a challenge.

The challenge doesnt complete either. I've put up another bug report for that.

Its annoying as I can't progress with campaigns or missions now.

I have similar bug. Can't complete mission to equip offense artifacts of 4 stars or greater on a rare attack toon. I have a galaxy note 10+. It is up to date. No reply from plarium yet. I spent a lot of silver trying.  I would like to get the legendary  toon for completing all the missions in the chain. Maybe some of my silver back too. The screen shot option seems bugged in game when I sent my bug report also.