Adding more items to smelt into a Furnace at the same time an item is done smelting glitches the item count

When adding items to a furnace which is already cooking some items of the same type, the total amount of items is not shown initially.

What I expected to happen was the added items would go into the furnace with the already cooking items.

What actually happened was the added items appeared to disappear until the next item finished cooking, then the proper number of items was displayed.

Steps to reproduce:
1. Place less than 1 stack of cook-able items in furnace (eg. cobblestone).
2. Put in fuel.
3. Add more of the same cook-able item to the top slot of the furnace.
4. Observe that the number of the cook-able items does not change immediately but instead changes after another item finishes cooking.