Iron ore in a furnace with fuel does not do anything, when the items should be smelted into ingots. Do not yet know if this is exclusive to iron, but both furnace and blast furnace do not seem to smelt on my server's world.
Aternos server running 21w06a 2GB RAM. World optimisation is turned on.
Edit: Made a new world and iron is smelting in this one. Bug appears to be intermittent between worlds: either the world will have smelting capabilities, or not. World was deleted and I cannot seem to reproduce the issue.