I decided to check out some of the other dungeons and found a similar problems.
The Scorpion Den (Sepermeru Silver Mine)
The opponents stop moving after a few steps.
Both bosses not only stopped moving, but also stopped attacking the player and his companion.
The Well of Skelos
The follower had a problem with teleporting. If a player has strayed too far from the follower, that follower has not teleported to its owner. The follower had to be led slowly through the dungeon.
When exiting the dungeon, the companion did not follow the safe path to The Shattered Stairs. The follower ran straight instead of following the thin, winding road. He lost 5,000 health points while running through the lava.
Now he rests wrapped in bandages and drinks a lot of aloe potions.
Visiting / farming broken dungeons also has its advantages. Any weapon and armor should be enough to kill all the bosses inside. In fact, even naked with fists is enough. That means free materials, recipes and legendary items - over and over again.
The archers standing by the main opponent in the cave didn’t attack until I got very close to them.
My companion refused to come near the archers (as if he was afraid ).
The Temple of Frost
The dungeon works fine but…
Hidden dragon boss stabs the follower undermesh and loses interest in him (after a few tries I gave up and left the beast alone).
The Unnamed City (location)
By the way, I checked all bosses in Unnamed City, there was only the problem with dragons just like the boss in the Temple of Frost… and something extra.
The Red Mother (world boss) - She also stabs the follower undermesh, but not only that. She is so large that when trying to hit the player, the attacks reached far behind his back and did him no harm.
You are talking about the serpentman-leader on the throne behind the forcefield?
He always died fast, for as long as I can remember. Probably from exhaustion due to summoning all those serpentmen-ghosts.
First time I met him I was quite surprised, I expected a tough fight, but he just fell from his throne. Nice idea, though
The serpent-man boss, the denegerate should just collapse and die by. Some times though he sits on the throne sideways and after the ghost-minion fights he attacks.
Fun fact he is sometimes reachable while behind the forcefield ( sometimes he will not be where he is supposed to and too close to the force field ) so I beat him to death ( it took something like 5 full predatory light combos )
He used to get out of his throne and fight after all his ghosts were destroyed, was a good fight even but he was broken quite awhile ago, its been reported months ago but as with most bugs these days, just put on a list and forgotten and he stays broken.
I mean really new players must love all the broken bosses, its great way to get players to talk about your game, only talking how its so buggy is not the best way to get word about your game out in my mind.
I set my companion to shoot from a distance. The horse was running around. The boss threw himself on the stomach several times before he died.
@Halk please provide more details. I will try to recreate your issue.
I’ve seen a few bosses lose interest and regenerate health (immortals), but if you go far enough to give him a chance to reset (or return to the spawn point), the boss will start working properly again.
Interesting - seems to be a difference between server and singleplayer - I just went and did the Dregs a couple of hours ago in my singleplayer game and it was behaving fine (spitting, launching out of the acid etc). I’ve definitely experienced it ‘going dormant’ previously (every time a couple of months back), so it’s not like the error doesn’t pop up in singleplayer, just doesn’t seem to be happening now. I wonder what’s sufficiently different between server play and singleplayer?
Edit: It was only me, and no follower - I wonder if that makes a difference? Maybe it gets confused by multiple ‘equal value’ targets?
Oh wait when I did the abysmal remnant on my private server it also kept losing aggro! It wasn’t regenerating or at least not enough for me to really notice but it did keep turning it’s back to me and getting angry at the wall instead
Solo with no thralls or friends… They quit when the update dropped 💁 But not a targeting issue unless it was aggroing on a skeleton behind the wall?
Not one of my problems anymore though! I hit (or more bashed) the uninstall button earlier today Gold luck though, hope it gets fixed at some point
As for losing interest in the player, bosses seem more focused after the patch. I tested a few cases, even if the boss couldn’t reach me and I didn’t attack him constantly, the boss wouldn’t let go (I waited 10 minutes doing nothing at all and the boss didn’t lose interest in me).
The Sunken City (dungeon)
I didn’t notice any problems in these dungeons:
and armor 
should be enough to kill all the bosses inside. In fact, even naked 
with fists 
is enough. That means free materials, recipes and legendary items - over and over again.
Palace of the Witch Queen
Xalthar’s Refuge (cavern)
).
The Temple of Frost
stabs the follower undermesh and loses interest in him (after a few tries I gave up 
and left the beast alone).
Skittering Cavern
I will try to recreate your issue.
Gold luck though, hope it gets fixed at some point 
As for losing interest in the player, bosses seem more focused after the patch. I tested a few cases, even if the boss couldn’t reach me and I didn’t attack him constantly, the boss wouldn’t let go (I waited 10 minutes doing nothing at all and the boss didn’t lose interest in me).