why do some essences just never die?

It will be dps down and then at about 25% it hp bar just NEVERS move.
It isnt healing it isnt immune to damage it just isnt moving.
The is nothing by it.
The map mods don't have anything weird.
if I run away to stop dpsing it isnt healing or anything like that its just not dying.

Sometimes there are certain mods stacked together on the mobs making it harder to kill them.