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.
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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.