This has been happening at low temperatures (so < 2000 as that's what it reaches before the first ingot is smelted from cold) whilst it's also happening at high (6000). Tested it at a few to see if temperature has an effect on it, and it doesn't seem to from my testing.What is the temperature in the oven?
Sometimes it seems like it smelts a 2:1 iron to steel ratio, sometimes it's a 1:1 iron to steel, and others it chews up 3 stacks of iron with no output whatsoever... >.>