Coal Block Furnace bug

Hello.
I was playing Minecraft with my friend and i was cooking some food with coal block and found a interesting bug. I logged out while cooking and my coal block ran out of fuel a little bit but when i logged back on it was full. (You will se it in the video bellow.)

What I expected to happen was...:
I was expecting it to be like this https://gyazo.com/68a7583e940ccf124d6158bbfc747200

What actually happened was...:
Actually it was full after i logged back on.

Steps to Reproduce:
1. The video of me showing the bug: https://www.youtube.com/watch?v=JUN5zTwufas&feature=youtu.be