The bug

When you start smelting a smelt bar is filling and after finish produces the output.

You can add manually materials as well as fuel and it does not affect smelting bar progress.

However when you add a material to already ongoing smelting using recipe book then the smelting progress bar resets.

Steps to reproduce

1. Ensure the terracotta smelting recipe is unlocked:

   /recipe give @s terracotta

2. Give yourself two clay blocks and a piece of coal:

   /give @s clay 2

   /give @s coal

3. Place and open a furnace, and open the smelting recipe book
4. Place the coal in the fuel slot
5. Click on the terracotta recipe
   → One clay block begins to be smelted
6. Click on the terracotta recipe again
   → The second clay block is added to the ingredient slot
   → The smelting progress is reset