Items that are unsmeltable (like leather armor) can be placed in the smelting (top) furnace spot. This is inconsistent with other mechanics since it is currently impossible to put non fuel items in the fuel spot of a furnace, so it makes no sense why it doesn't work for the top slot. This bug has been in the game for FOREVER and has been marked as fixed in java but is still in bedrock. Since it is a relatively simple fix I don't know why it's still in the game as it limits some sorting and is really annoying.