Hivebrain Gland bug with Esh being immune with infinite phase

Seems like if you deal to much damage to Esh (not the shield guy) in the hive brain encounter, she/he will become immune and it means that the encounter is over, it starts when phase 1 is over, so before sandstorm thing, please fix guys ;) not the first day bug

Is this the reason I can not kill it?

I thought it is just super difficult to do, since it seem almost immune to damage...