I completed the 1st phase of the headshots challenge, and the challenge just disappeared, I can't complete more phases :(
We're aware of a bug with this week's Legendary challenge that makes it disappear as you complete each stage but re-appear as you get closer to the next stage's completion.
The details on each stage's threshold is on our Trello Board here: https://trello.com/c/91CeSQDL/1583-legendary-quest-get-headshots-is-not-always-visible