This is an issue that I've tried to figure out in the past few weeks, and I think I finally figured out what it is.
With a stoked furnace, if you have 4 smeltable ores and a gemstone/moonstone and the smelter chooses to smelt the gem/moonstone, it will not smelt the 4 ores afterward. It gives the "Just Chilling" screen. Dropping an ore does fix it and pushes the smelter to smelt the 4 ores, but in some cases this isn't really viable, so it takes up space in your cart's return. (For instance, I'm not going to drop a single gold ore if it's the other ore I have in my cart). It seems smelting the gemstone/moonstone makes the cart reset into thinking you need 5, but if you have 5, it will still smelt 4.
I don't currently have a screenshot as I just sent my wolf out, but I can get one soon enough as it happens quite often.
[e] I have put it into a gif:
https://i.imgur.com/BiymFEJ.gif