I think I found a work around for the missing challenge mote for dancer in the dark achievement. If it is missing simply log in a different character and then go back to the previous character that’s missing the challenge mote and try again. This always allows the mote to show up again.
Worked for me as well. I closed the game, logged into a different character, then logged back into the character I wanted to do the achievement with and the challenge mote was there.