Instead of the hopper putting fuels in the fuel slot of a furnace, it puts it in the slot where the items that need smelting/cooking go.
Comments 4
Ah okay, I still think that putting the hopper in any orientation should put the items in their appropriate slots in the furnace.
Some items don't have an 'appropriate slot'. E.g. wood blocks can either be used as fuel or smelted to charcoal. So which slot things go in has to be determined my where the hopper is, not what the item is.
You must to put the hopper behind the furnace in order to put the fuel in that slot... 😉