Maybe invalid metadata check, since was used meta item as input.
https://youtu.be/28lJCofSJvw

Suggested fix: #179 (I made a mistake, will update later)

I have played around and found out:

• it is reproducible in steam furnaces as well;
• it doesn't seem like metadata issue: different materials are also affected by this (putting dark ashes and vanilla pork together works too)

Also #137 might be related

The problem is taking effect only at E-Furnace