Triggering Mission Bug (months-long issue)

Hello, I am posting this in the big thread and I have posted this twice in the general discussion forum over the course of two months. Apologies for the spam post but I am somewhat desperate for help. Forgot to mention that I am playing on Xbox: 

I have been trying to complete the Eelside Down step of the Bag of Tricks mission from tour 2. I believe the problem arose when I had to force quit the mission without saving anything. Since then, when I load the game and attempt to activate the mission, the purple waypoint marker disappears when I am 10m out, and I cannot play the line challenge required for the mission to be considered completed. 

I have attempted to fix this bug by randomly teleporting to players around the map, restarting y game and system multiple times, and starting co-cop/party mode but this has not resolved the bug. 

Thank you all for your time and again, apologies for the spam.