Furnace exploit?

Furnace Exploit?

So im just smelting some iron in my furnace and then when I go to the ps4 home menu (which I did because I had to do something and I thought it would pause everything) so when I came back all the iron was smelted... but only 1 coal was spent. Im not 100% sure if this is truly an exploit or not. But either way, please try to do something.