I am also experiencing this issue. In my case, it's happening in the Seeker Slayer Survivor arena.
In the Maerwald fight, Konstanten had over half his health left and no injuries, then suddenly died and was deleted from the party. I tried to find what had happened to him in the combat log, but it looks like he only got hit by a few magic missiles. Here are my savegame fiels from before and after the fight.
Before:
https://www.dropbox.com/s/lbsysg6axdrhizi/Litni%20%28c4541131-fa71-4c99-ab9a-37190b115afd%29%20%28LAX-123ABCDEFGH%29%20autosave_1.savegame?dl=0
After:
https://www.dropbox.com/s/5jnnhcebjauy1a6/Litni%20%28c4541131-fa71-4c99-ab9a-37190b115afd%29%20%28LAX-123ABCDEFGH%29%20autosave_2.savegame?dl=0
I hope this issue gets prioritized soon. Having a companion with story content get randomly deleted is a pretty heavy roadblock to playing.
