I just tried the Harbinger mission to get all the feathers to complete my lorebook. However when I went to get the fourth feather it simply did not exist. I consulted numerous guides and found that I was not the only one experiencing this as many others have not been able to complete this Triumph in recent months. I myself was on the second week rotation and could not locate feather #4 despite it supposed to be where the hobgoblin spawns upon entering the Taken portal.
Unfortunately I've been able to crosscheck this issue with both Week 2 and Week 3. This is in fact a glitch and not a case of me looking in the wrong spot.
Last week during the week 2 rotation it was the hobgoblin feather that wouldn't appear. This week on the Week 3 rotation the feather at the train tracks is gone. So the glitch preventing the completion of the Lightseeker Triumph is still ongoing.
Actually it is right in the little alcove next to the door. I had this issue until I found it. You do not go inside it is outside the door in the left upon approach look there.