Completed Spirit of Fire, Last Ruin, Age of WInter with a friend in co-op. Made it to Shattered Road. I have no NPC here to talk to but see a text bubble as if there is supposed to be one. I cannot empower rifts, but he can. I still have the quest to conquer The Last Ruin timeline, even though we finished it together, and it ate all the stability upon completion.
Update: Played through the entire Last Ruin campaign again. This time the Harbinger Eye dropped, and I was portal to the shattered road where the NPC was waiting. Neither of these happened last time. I can now empower monoliths correctly.
So it appears that the bug is that the Harbinger Eye did not drop for the non-host in multiplayer. My friend is the one that opened the initial quest from the monolith and had no problems.
Thank you for continuing to look into this. I would hate to go through an entire empowered timeline, finish it, and lose all the stability without getting the progression item each time in co-op.