Black Crow Boss bug.

Just had a bug with Black Crow boss during mapping. During the 2nd phase when he enrages, I attacked him until he had 5% of hp left (didn't deal crazy dmg to it so he didn't immediately fall to 5%, aka he should've had enough time to get the bell for phase 3). Then he became completely invulnerable (couldn't deal any dmg at all) and he just kept attacking me, even when I moved around with him, he didn't take the bell.
I have an Ice Strike monk so he was frozen a few times, but I didn't stunlock it or anything.

I attacked him when he was running towards the gate after phase 1, so maybe this bug still persists even after over half a year of dev time (I can recall the Black Crow being buggy during launch as well?)?
This is a pretty major bug because if I had more modifiers on the map I would've lost a map due to a stupid bug, which is unacceptable.

Edit: I killed it during the second attempt. I didn't attack him when he was changing phases and I guess that avoided the bug.. :/