I don't know if this is a bug or not but I've paid to stoke the furnace and have easily made over 10 bars so far and the number isn't going down. I haven't been able to pay for a new stoking for 4 days by now and I was really hoping to.

Something that might be affecting me and, if it is, I guess I just need to know, is that I'm doing a 'gem hunt', where I'm filling my 30maxed out cart half with gems and the other half with ore before I go back to the furnace. Could that be affecting it? Do you have to have a full cart of ore to be able to 'use' the furnace or something?

I'm getting this issue too and it seems some ores don't get smelted in 4 when the furnace is stoked. I had 4 mithril and 8 silver. The furnace had 4 or so charges left so I figured it will perfectly smelt all those ores. Instead, it smelted 5 silver and stopped. I dropped the remaining 3 silver and that restarted the furnace to smelt the 4 mithril.

This bug is persisting, and as others have mentioned before, the best way to combat it is to have at least one piece of ore you wouldn't mind dropping to push the furnace to stoke if you have four matching ore left in your cart. I.e., I usually farm gold ore, and I'll keep at least one lesser ore (silver, etc.) that I can drop just in case I get stuck so I don't have to waste time and cart slots going back and forth between high level mining areas.