When you place a hopper under a vanilla furnace to extract the output of smelted items, it pulls the fuel (coal and charcoal tested, probably all fuel as well) out of the furnace making the furnace and hopper pointless.

Steps to Reproduce:
Place hopper under furnace, put fuel in furnace, watch it dissapear.

Technical Information:

• Minecraft: 1.17.1
• Fabric API: 0.11.6
• Industrial Revolution: alpha 12
• Modpack: no other mods.