Mm, I’m using gregtech so I can’t smelt logs into charcoal in the furnace. It is a regular old Vanilla furnace I’m using, minecraft:furnace and I don’t believe I’ve anything installed that modifies the furnace code.
What about limestone as a test? I can smelt limestone into smooth limestone (1735:7), so that’s sitting in output, now if it kept bugging I should be able to smelt limestone cobble(1735:1) directly into smooth limestone but it doesn’t smelt it until I remove the smooth limestone.