When my wife and I play couch coop and hit max threat in helltides, we’ve never been able to kill the hellborne; there are two, and they always kill us first (despite us quickly deleting all other threats in the game). This has been true for the entirety of our leveling, now 70.
When I co-opped with my dad remotely, not locally, when our threat maxed out there was only one hellborne and it was easily dispatched.
I suspect there may be a bug with hellborne power scaling for couch co-op players.