Why the Fortress Boss' Poison won't dispear?

T16 waystone with 24% delirium or so, can't remember clearly.

I picked three seals and released Boss, start doing damage and then she hide as always, then posion gas start spreading from center well, then I found her and doing damage, for like 3 ~4 times.

The poison gas never stopped, and finally I was poisoned dead.

Why?