@ubi_plush

Took a screenshot of the window.
From everything I can tell, I have the most up to date version of the game. Also, for further clarification, I play on PC via Ubisoft Connect. And yes, I do have a Cloud Save as well. I imagine that will allow your team to access said save file to test it out themselves. I can also assure you my specs are well above what the game needs, so if we're narrowing things down to isolate where the bug is that's one other variable we don't need to worry about. Seems like a logic error with the A.I. for Fenrir. But that's just my guess as an Indie Dev. Prolly something to do with the tracking feature. The one video has him stuck up on the hill underground. He stayed there the entire time until I moved to the far edge, but only to a specific spot which was the closest point adjacent to his cave, and that prompted him to switch over to doing that endless tunneling. Judging from the behavior, it seemed like the A.I. couldn't detect where I was, so maybe something with the Nav Meshing? I dunno.

@ubi-borealis
So yeah, I just ran into this bug.
I'm currently on my 5th attempt at doing this fight, each time I've had to reload a prior save because it keeps glitching after the second binding for one reason or another, always associated with Fenrir diving underground. Here are two videos I took;
Video One:

Video Two: