The triumph has never been bugged. What has and still is bugged is that two of the Harbinger weeks only spawn 4 feathers. Only the first week's Harbinger spawns all 5 feathers which is required for the Lightseeker triumph. There are three different bosses for Harbinger and they rotate weekly. So only the week 1 boss version will spawn all five feathers. Week 2 and 3 only spawn 4. If you didn't get the triumph then you were likely in week 2 or 3. The week 1 boss is the Taken Knight. The week 2 boss is the Taken Captain. And the week 3 boss is the two Taken Centurions.