Imho, underground and in darkness is when you would want it most, not a shrinking radius.That is more than a fair complaint. Maybe the radius shrinks if underground?
My best visual example is a group of adventurers in a dark cavern huddled around the cleric’s twilight aura, as he or she boldly presents her holy symbol. They look high and low, preparing to be attacked yet not knowing from where the enemy will strike.
maybe that’s too dramatic, but it’s where my heads at.