Originally Posted by catclaw
This happened to me but I think it's a bug for my situation. But I read somewhere that if you go with Karlach to Avernus (w/Wyll or w/o) it overrides the ending with Halsin. Maybe this is a bug too...? I'm not sure. Or maybe it make sense you wouldn't have spare time to take to Halsin real quick anyway since you yeet to Avernus on the spot.

No, I finished the game before the final Karlah was added.
Then the error worked if Karlah became a mind eater.