If you have a furnace smelting an item and you save and go to title when rejoining the game will show the fuel bar of the furnace as full. The bug is only in that graphic because after 8 items smelted with coal the fuel bar will start to rapidly go from maxmimum to 0 while the last item is smelting.

Also after/during this bug after a while if you have the furnace ui open you will start hearing the nether loading terrain sound periodically.

Image taken at 17.41.33 is when you sart smelting an item, 17.40.53 after a while that you are smelting, 17.40.53 is after rejoing the world after having pressed save and exit, 17.41.33 is what happens when the last item is smelting (it is the 8th since i used coal) as you can see the fuel level is going from 100 to 0 with le last item whereas it should have done it gradually while smelting the eight items in total.