I made a minor PR that would fix this (see #12), but I figured I should open an issue for formality.

The Create mod metal mixing recipes that are added were apparently supposed to require heat, but used a misspelled tag heatRequirement (should be heat_requirement).

Thanks for your work!

No problem at all! I'm gradually becoming more interested in mod development, so anything I can do to fix other things in mods helps me learn too.