Delirium Fog doesn't propagate into the Riverhold map boss arena

I'm using the Clear Skies Delirium Tablet with the "Isn't it tempting?" Atlas passive. After fully clearing the map before killing the boss, the Riverhold boss didn't drop any extra currency or gold, a statistical impossibility. I noticed the fog effect wasn't playing in the arena, so it appears that Delirium wasn't active there, even though it should have been.

Just outside the arena, you can see the fog effect, while inside, it is gone.