Delirium Fog not disappearing... kinda

Got Delirium Mirror spawn in a Maze map. Run through it, and fog began to disappear as I was fighting the boss (Vaal Oversoul). Killed the boss before timer ended, and as soon as he died, timer disappeared.

Meanwhile, Delirium music is still playing in map, there's still particles falling down, and everything is grey-ish. There is even a button to stop Delirium encounter. The button worked, and I got my rewards.

I do not use the Unending Nightmare keystone, and I did not use any Delirium orbs on the map.