I just made it after 10+ tries on that bugged boss.

Here's how to do it:

Bug seems to occur as soon as he enters his ascended form. DO NOT damage him while he is transforming!

First Phase is easy. Just bash him with everything you have.
If he bugs out while transitioning to the 2nd or 3rd Phase (because he starts Phase 2 while ascended) Bad Luck. Try again.
In 2nd Phase damage him in his ascended form until he turns normal again (first shield bar down).
Now damage him until he is about to get staggered (but don't actually stagger him!).
Now run around, kill adds and wait until he enters his ascended form again.
He will do it after a while.
After he transformed: DO NOT Damage him for another 2 seconds or so. Wait for him to start moving again.
Then you should be safe for that turn.
Rinse and repeat for Phase 2 and 3 until he's down.
Took me a while to get it right and yes this prolongs the fight for some time but anything is better than not getting it done at all.

Let me know if this works for you as well :).

Screenshot of proof:

https://i.postimg.cc/hP9dmLX2/Unbenannt.png