In single player survival, when using a bucket of lava for fuel in a furnace, the furnace flame animation both in the furnace GUI and on the furnace block itself never goes out. It remains burning even after an item has been cooked or smelted. Removing the cooked or smelted item or removing the empty lava bucket from the furnace does not change anything. However, if you REMOVE the empty lava bucket, you can continue to cook or smelt things without any fuel being in the bottom block of the furnace GUI as shown in attachment 4.png.

STEPS TO REPRODUCE:
1) In single player survival, create and place a furnace.
2) Obtain a block of cobblestone.
3) Obtain a bucket of lava.
3) Click on the furnace to open its GUI.
4) Place the bucket of lava in the furnace at the bottom for fuel.
5) Place the cobblestone block in the top to be smelted.
6) After the smelting is complete, observe that the flame animation in the GUI and the flame animation on the furnace block itself remains continuously on.