Hey! My fireteam was experiencing a similar issue, and we somehow narrowed it down to the Gunpowder Gamble graphical glitch. The one that has the grenade permanently attached to the Hunter. Did you have a Hunter in your fireteam? If so, that may have been what happened. Some other weird interactions with the tether is that it can't go through barriers, sunspots, and also breaks when the Hunter with the Gunpowder Gamble grenade stuck to them. We fixed it by having the Hunter (me) leave the fireteam and rejoin. I was able to do that since I already had Divinity, otherwise my progress would have been reset, so watch out for that. Hope this helps!

Im guessing you are doing the final diamond puzzle. If you are jumping through the tunnel, try going around instead. I think sometimes when you touch the rotating panels inside the tunnel it screws around with the tether's detection. For, beasting the harpies make sure your team has as much resil as possible and use two concussive dampeners, it helps drastically. Lastly, be careful jumping on and moving over stares, that can mess with tether as well. When you have the two people connecting the diamonds on the staircase just take it slow moving up the stairs nice and easy. Ur not timed, take it slow. One last thing, don't block with a sword while tethered because it will break the link. If you make it to the boss, good luck I hope you finish. 👍😊