Once I smelted a bunch of glass using lava buckets. I decided to get more lava from the nether as my lava farm wasn't fast enough, but when I came back I noticed the furnace burn time wasn't as I remembered. It was less than half way before I left, and when I came back, it was showing a full burn time.

I decided to look at the NBT data of the furnace. There is a Short Tag for the burn time (the time it takes for the furnace to take the next fuel) and the cook time (the time it takes for the item to cook). However, there is no tag for the initial burn time that the fuel item had.

How to reproduce

1. Place a furnace in your world
2. Cook a bunch of items in it using a powerful fuel source (like coal blocks, lava, kelp blocks, etc) until the furnace fuel time is depleted over half way.
3. Log out of your world and then log back in.

Observed result

The furnace GUI will show a full burn time for the furnace. As it does not remember their initial burn time.