Was doing a Sinkhole 8. It’s the eel boss version. And Brann kept pulling him further and further down the hall until boss would respawn. I had to basically not follow him and let him die multiple times just to keep boss from despawning. I use him as a tank, but this isn’t the only dungeon I have had issues in. I think it’s Spiral Weave or one of the spider delves he also reset boss on me before.
Stand right by the treasure room door and DO NOT MOVE from that spot. It will force Brann into a position where he won’t pull the boss backwards and out.