Here's what i've done before it happened

(P.s. this happened simultaneously on all 4 furnaces i was using)

1. The "fire" animation of the furnaces was about to run out. Also, there's no fuel left in the box below it. The fuel I used is a block of coal. There's also no item being smelted.

2. I added a cobblestone on each of them to make use of the remaining "fire".

3. Went into the nether.

4. Came back into the overworld and the fire animation is filled again. Which supposedly should've ran out.