EDIT: Search function did not show this bug on searches before I wrote this report, have since stumbled upon. Please close/mark as duplicate, thanks

Furnaces will not smelt/cook items unless there is extra fuel available.
This mean, for example, 9 pieces of coal will be required for a stack of items, 1 per each 8 items + 1 more to keep the furnace "working".

(see attached screenshots, one shows the furnace becomes "fueled" and the second shows that an extra item is required in order for the furnace to begin processing the item)

 To reproduce:

Place valid smelt/cookable item in a furnace and add 1 fuel item, no progress will be made until more fuel is placed in the furnace.