What I expected to happen was...:
I place a stack of a smeltable item (Cobblestone, Raw Beef...) in top furnace spot.
I place a single Fuel item (coal) in the bottom fuel spot.
The single Fuel item (coal) is consumed. The flame graphic appears
Over time, I get the smelted/cooked item(s) (Stone, Cooked Beef...) in the right slot

What actually happened was...:
I place a stack of a smeltable item (Cobblestone, Raw Beef...) in top furnace spot.
I place a single Fuel item (coal) in the bottom fuel spot.
The single Fuel item (coal) is consumed.
The flame appears, but the smeltable item fails to smelt/cook.

Steps to Reproduce:
1. Place a stack of a smeltable item (Cobblestone, Raw Beef...) in top furnace spot.
2. Place a single Fuel item (coal) in the bottom fuel spot.

Note:
This only appears with a single Fuel item. If a Stack of fuel is placed there is no issue