Using multiple furnaces being updates constantly deletes the coal/Fuel within the furnace.
A row of Furnaces where the item is placed within them using a hopper and mine cart with a chest, ad well as a row of hoppers below to place the output from furnace into a second minecart chest.

I expected to place items into mine cart chest that will place items to be smelted into the furnace where they will be smelted by them with fuel thats already within the furnace. The output would then be placed in a hopper and picked up by a mine cart chest.

It worked exactly as i wanted however several of the furnaces fuel would be automatically be deleted. It appears when the mine cart chests are moving most quickly above the hoppers is when the furnace deletes the fuel.

Steps to Reproduce:
1: row of Furnaces
2: Hoppers Above all the furnaces
3: Rail on all the hoppers with Power rails at the end to send it back the other way(so it loops over them all)
4: Hoppers bellow all the furnaces
5: same method to collect the output of the furnace with mine cart chest.
6: Fuel in all of the furnaces
7: add item to smelt in top mine cart chest and set it back and forth