After entering an older test world from a previous update I made a flying machine and used a flint and steel to light the top surface of an observer. After the pistons had moved the fire appears to have persisted. I placed a block underneath, as seen in the clip, and removed the block after. It remains. I hit the air to remove it and nothing happened, but placing a block underneath allows you to remove it. Leaving the world in anyway does not remove the fire. The world has normal settings other than fire spread being off.