Tier 11, tank Brann, Sinkhole, kill Cobyss/recover 8 Stolen Relics, kill Cragpie scenario.
I pull mobs, and Brann just stands there. This is especially bad any time we’re on a slope (which is a great deal of the dungeon). It’s like the slightest elevation change confuses him.
I just completed the same delve with him as tank and my character as healer. He was all over the place, but I never had issues with him not attacking. Was your character also in heal spec? I was using underwater breathing potions so I didn’t need the air bubbles, I’m not sure the bubble would affect this or not. The action button ability didn’t seem to work as expected, either, so I never used it. Always fun to run into these glitches. 
I posted about this on another thread, as I had this happen while healing Brann’s tank spec on the final boss of one of the kobyss delves. Definitely some issues.
Heal spec, underwater breathing potions (I hate the bubbles). I never used the extra action ability, because I figured it was a death sentence.
I was using ranged attacks. Were you in melee? I suspect there are fewer glitches if you’re in melee, whether as heals or another spec, just because Brann won’t be at a distance, so you can sort of position him.
I also noticed having Leap of Faith on a priest is great for pulling Brann out of the bad, because he doesn’t have any sort of instinct for self-preservation. Sadly, druids have nothing similar, so Brann just… stood in the bad and died a bunch, too.
Yes! I was on mistweaver monk, so I was almost entirely in melee range (except when he stands in bad and floats in the water off the side of the cliff lol). That sounds like it could be what saved me. He still acts pretty crazy even so. He was standing in webs in Earthcrawl Mines and did not dodge the mine cart mechanic on the last boss. Leap of faith would be great for those scenarios. But, I guess I’m glad I didn’t have this particular issue (yet).
That last boss in Earthcrawl was a nightmare with tank Brann! Like… dude, if you just stand diagonal to the tracks and take one step back, the carts don’t hit you.