Delve destination nodes keep going dark

I get to a destination node with my handy crawler and he sits there keeping the area light while i fight the mobs but then the area periodically flickers dark for a few seconds at a time. It's annoying but also it stops me doing damage while the light is out

Is this supposed to happen? Am i missing something?

Thats a rare mob with some "powers". Darkness in delve makes mobs immortal. They need to be in light to be damaged.

Use flares in such moments.