In some instances, where lava had been in the world in the past, there is a single block area that glows at night. You see nothing there, but you get injured if you walk into it, and you start on fire if you stand in it too long. It has the exact properties of fire, but it's not there! What is happening here? Was fire not added, or was it just made invisible?

P.S. Some of the "invisible fire" is two or three blocks off the ground suspended in air.

(Using Kindle Fire HD 7)