During last week’s Necrolord assault, I chose the final champion that I needed to complete the “This Army” achievement and the related “Breaking the Chains” meta-achievement. I had the champion with me throughout completing the quest requirements. I completed all the other quests during the assault and then turned them into Draka. I was not awarded the achievement or the meta-achievement. I believe that their may be a bug related to the Kevin mount that may despawn your champion and not allow you to get credit for the achievement. The game masters were/are not willing to correct this issue and award the achievement and since there is such a long gap between when all of the required assaults and quests are up this is going to lead to several upset players if this issue is not resolved.
I wonder if a workaround for now would be to only accept the “You and What Army” quest and its relevant champion and turn it in before accepting any of the other quests, including the Kevin quest? Or turning it in before mounting Kevin? I think you could also abandon the champion quest and reobtain it, although having to start over is not the greatest.