I do not know if this should be a bug or a feature request.
anyway, I was making a block that generates tree items. this is similar to the furnace. And I was wanting to use hoppers to input into the input slot and (when the hopper is placed below the block) only take items out of the output slot. I then went and tested it and the hopper inputs and outputs to any of the slots starting with slot 0.
In minecraft itself, the hoppers can input into the furnace only to the input slots and take out from only the output slots.
It would be really nice if this was fixed and if it was not even in the program, to begin with adding it to mcreator.