The naturally generated furnaces in a village do not work. However, if you break the furnace, take it as an item, and place it, it will work as normal.
Furnaces generated in a blacksmith (village) do not smelt any items
World-gen furnaces do not work
Naturally Generated Furnaces Wont Cook Things
Naturally generated furnaces dont work
Block Entities are not upgraded properly when upgrading from 1.12.2
Liquids in generated structures don't update