Bug in the Heliophage

so was playing the heliophage in splitscreen and it went all really well but at a certain moment we fought the boss with the shield and the 3 guys with lasers on their back. We both got knocked off and needed to respawn when we got back in the arena we found that one of the guys shielding him had teleported outside the map in the direction of rendain and he was completely out of our reach.
If you go towards rendain you can get further then you think but this enemy was way way to far to kill.
Not sure what caused it but we couldn’t finish the mission because we couldn’t hit him in anyway.
And its the last mission my girlfriend needs to complete and we didn’t want to do it again cause it already took us 40 min to get to this point.

has anyone had this problem?
And I made a clip about where he was if that is any help to the devs or what so ever