The bug

When you use different smelting recipes to obtain the same item, only the last recipe used takes effect.

How to reproduce

Recipe

1. Install the attached data pack
2. Place a furnace and smelt a piece of rotten flesh
3. Without removing the anvil from the furnace's output slot, smelt a piece of stone
4. Take out the two anvils and see that you only get one smelting recipe

Experience

1. Install the attached data pack
2. /xp add @s -100 levels
3. Place a furnace and smelt a 32 pieces of rotten flesh, then take out the resulting 32 anvils. Note that you have about half a level of xp.
4. /xp add @s -100 levels
5. Smelt 31 pieces of stone, then take out the resulting 31 anvils. Note that you have about 3 and a half levels of xp
6. /xp add @s -100 levels
7. Smelt 31 pieces of stone, then smelt one piece of rotten flesh, and then take out the resulting 32 anvils. Note that you only received half a level of xp, whereas from step 5 you knew to expect at least 3 and a half levels of xp.