Description of the issue:

The recipe serializer for metal press recipes currently uses a stack instead of an ingredient for the mold, which prevents the usage of tags and other custom ingredient serializers.

Versions

Immersive Engineering: 1.16.5-4.2.4-134

Yep that's intentional. We use molds for caching and access recipes quicker. Ingredients are not hashable nor comparable, making them useless as a map key.