EDIT:

Workaround confirmed :). Got it done this way after 10+ tries.
Details:
https://wolcengame.com/forum/bugs-issues/bug-reports/confirmed-workaround-act-3-boss-bug/

Hi There!

I think I found some sort of cheesy workaround for the Boss in Act 3.
Phase 1 is completely normal. Just hit him with everything you have.
As soon as the Boss goes to Phase 2, DO NOT go all out on him while he is in his "normal" form.
I think the bug happens when you "stagger" him in his normal form (Red shield thing above) shortly before he's about to turn ascended again.
I avoided staggering him in Phase 2 and successfully managed to get to Phase 3 with this technique.
I know this prolongs the fight but I think it's better than resetting every single time he acts like a stupid NPC.

Sidenote:
You don't have to quit the game to reset the fight. That round thing on the bottom right corner can be damaged and spawns adds after hitting it hard enough. Just get killed by these adds and you can start again. This way your Primordial Essence is also conserved.

Let me know if this works for you or if it's just a stupid coincidence :)!