Furnace Not Smelting Sulphur - Rust Ez Survival

Not sure how temporary this is...
When I was trying to smelt 6,319 sulphur in 4 stacks (no fuel/VIP) in the tiny furnace, the furnace would stop.

If I smelted with the window open, it seemed to work fine, but as soon as I closed the window the furnace would stop smelting within seconds.

I didn't click the two red buttons above the stack modifier. I did toggle on/off to see if it would help, but it didn't.
I broke the furnace and replaced it, to no avail.

It's not a game-breaking bug, as I can use big furnaces, but I just thought I'd mention it.