I tried to smelt 64 cobblestone with 8 charcoal but came back to only 63 smooth stone & 1 remaining cobblestone. Experienced the same issue smelting logs into charcoal. Both times I either saved & reloaded or traveled outside of smelting range during the smelting. Tried a single charcoal and successfully smelt 8 items. Also tried a full stack with 8 charcoal while standing in front of the furnace the whole time and successfully had 64 output. The bug doesn't seem affected by the order that I put fuel or item to be processed into the furnace.

What I expected:
64 output regardless of saving/exiting/traveling (using 8 charcoal as fuel)

What actually happened:
Found 63 output & 1 unprocessed input item if the smelting was ever paused by exiting or by in-game travel. (All fuel spent) This happened both in a survival world with experimental gameplay OFF and in a creative world with experimental gameplay ON.

Steps to reproduce:
1) Place 8 charcoal as fuel & 64 smeltable items into furnace (either stack first). The furnace should now be actively smelting.
2) Save & exit, then reload the world. OR walk far enough away that the smelting process is paused.
3) Return to the furnace and wait for the smelting process to finish. The fuel will be spent and 1 item should be left unsmelted.