Furance Fuel Flame Animation doesn't Decrease with Lava Bucket

When I am using lava buckets to smelt sand into glass, I am noticing that the flame animation doesn't decrease if you have a second lava bucket in the furnace after it uses the first.  If I don't have anything in the furnace or another fuel type (i.e. coal, charcoal, wood, etc.) then the flame animation is present.  I am not using any redstone in tandem with the furnaces, just 8 furnaces (4 long x 2 high).  I have repeated this about three times just to make sure it was a bug.