I got the Illusion a few days ago, learned it, but ATT still shows as not collected.

Have you tried doing a Shift+Click on ALL THE THINGS header? This is because Blizzard doesn't send out any flags to state you learned an illusion, but will check it off when you force refresh it.

It worked, thank you! :) I thought just by restarting the client it would update, but it looks like ATT needed a manual refresh.