Just got this same bug. Had to restart the game, but now I cannot access the servers anymore =P

I think, it kinda looked like this:
https://ibb.co/kg6hzgJ
Same goes for me, and for some other players. found 5 reports to that bug, till now.
First/original-Post was here:
https://wolcengame.com/forum/bugs-issues/bug-reports/boss-act-1-3a-phase-bug/
Maybe you want to post your case also to that thread, for a better recognition?

i am not sure if this help's, as i had to re do this fight 6 times since thats how many times he bugged! (patience was running super thin) .
But once he gets invulnerable, stay close to him, when the aoe falls, this way he will come out of his shitty invulnerable stage.
only thing is, he does that blast once hes ready to fight again, which can one shot if you dont dodge.

Hope this works for anyone with this issue, which is a fair few i have seen.

Somehow, it IS possible to get out, it seems - yes... but still, it's a annoying bug. But I think, the Devs first will fix the huge problems they got with the online-section of the game. that seems to escalate right now.