'Follow Lachlann' Not Completing, But Needed to Progress

Bug report number from in-game /bug command was 4,041,363,737

TL;DR
Follow Lachlann objective after opening Memorial Gate doesn't function as intended, playing coop, won't complete like it does in solo play.

Longer version
I was playing coop. Player 2 opened the Memorial Gate while I was gone. Couldn't get in at first, so we exited game and came back, then I opened it on my own and could get inside. Invited Player 2 back. But the 'Follow Lachlann' objective doesn't complete and start dialogue then a boss battle afterwards like it did for him when he did it alone. Just stays static even though all he had to do was enter the room, so I can't progress the quest. Does this even if I'm not in a party and go in there alone. How do I progress? Exiting game, and leaving the area and coming back didn't fix the issue unfortunately.

Thank you,
Player 1