Core Map Second Phase Bugged

Hi, wasn't sure if this was posted before but I just experienced a weird bug.

Shaper influenced Core Map with the following mods:

Are has patches of burning ground
monsters have 100% increased AOE
30% inc movement speed, 37% inc attack speed, 35% inc cast speed
monsters reflect 19% ele damage
Monsters have 65% chance to avoid poison, blind and bleeding.

Boss had received shaper buff and was more dangerous than usual.

During phase 2, rather than releasing the third heart, the boss burrowed and popped up beside the third heart and remained glitched and immune to damage (as seen in video link below)

https://gyazo.com/d9648f99f568094078102f788ac12ee9

Doesn't seem to be isolated to just that map - I was able to reproduce that on A4 Malachai (killed Piety with Bodyswap; stood there immune) and A5 Avarius/Innocence (took Innocence to 66% health with Ice Crash; would not enter minion phase). In both cases, I was using an AoE skill; the fact that the targets were cursed (via Blasphemy) may or may not also have something to do with it.

It appears that a workaround is to leave the boss fight and reenter. In the A4 Malachai fight, exiting via Portal Scroll and immediately re-entering was enough to force the game to proceed with the HP trigger event.