Burning Boxes only check for Air right before consuming a piece of Fuel. So the larger the Burn Time of said Fuel you are using, the longer it takes for the Box to extinguish.
The easiest Solution to this is turning your Coal Bricks into Nuggets before burning them inside the Box, or using a different shorter burning Fuel.
And yes you can just handcraft Coal Gems into Coal Bricks, which might be more convenient for the Nuggets, and can also be placed in World!
As for that Post I just quoted:
Don’t just do that to new people!
But hey, at least I found out that a lone Smiley on one Line is being increased to twice the size, so you are getting away with it this time.