Bosses like Gorgol Terror become invincible and stay that way

Since the start of 3.13, I have noticed sometimes bosses getting into a state where they stop taking damage, and not coming out of that state. Mainly bosses that transition from one attack pattern to another. It seemed likely while they were doing a big attack in their first pattern, if you dealt enough to carry the boss below their threshold for changing attack modes, they get stuck in the pre-intermission state where they are invincible, and keep attacking with attack pattern one attacks until the player dies.

It was easiest to reproduce 100% with Gorgol Terror in a t3 Azurite Fissue in lower Delve depths.

The boss comes down, yells, makes a flame breath attack, then chases the player. It is supposed to take flight when at 50% hp. However, if you deal 50% or more of its health in that flame breath, it chases you around, making attacks, and never takes flight.

The only workaround is to let it hit you for a while, then it can take flight. Unless that is done, it will chase and attack endlessly.

Here is a clip with the repro:
https://youtu.be/5ZS_jllDBQ4

I had one failed attempt in carrying the fight on what was definitely too long. I go back and in the second run you can see the boss goes invincible and stayed that way until I ran out of juice to keep distance and died. Went back and ran around taking and dodging hits after a short while of invincible boss, and it took flight. It was invincible in the third run less than the second.

NOTE: The video is being muted since I was listening to some music when I recorded.

I had a similar problem.

Low delve and I used vaal ground slam that could have probably one shot him. I did stun him on the hit. Since he won't go below half health, he stopped taking damage at half health and then attacked and killed me with nothing for me to do other than run away.

Every now and again I see it starting to happen (like their HP stops and I know the boss I am fighting should be changing phases/patterns), so I run off, avoid for a bit, take a hit or two... It sucks, but it is a bug that will persist until the devs recognize, list, and fix it. I gave repro steps and it is pretty obvious.