Not sure about hot springs specifically, but I do know that any given body of water that's fairly deep will freeze from the top down, usually with only a relatively thin layer of ice on the surface. So I would think that if a hot spring was gonna freeze, it'd form that same kind of layer. I never really questioned this, even as a kid, because I saw that pattern of freezing on my local lake. Figured it was the same principle in LBT. The surface of the water was cold enough to freeze, but not cold enough to penetrate much further down, thus thin ice.