This bothers me very much. Here's the situation: I'm walking around with a hot thermal stone, I enter the caves, thermal stone is suddently warm, I exit the caves, it's hot again.
This is annoying as when I have to spend a bit more time in the caves I either have to waste some fuel on a campfire or just let my thermal stone lose durrability. I wish its temperature in the caves would stay consistent with its temperature on the surface.