When Mystical World's copper is disabled in the config, Primal Core's copper nugget to ingot to block (and back) recipes do not work, but do show up in JEI. I also checked it to see if it was other mods' copper as well, and it didn't seem to affect Thermal Expansion or Immersive Engineering's copper. Reporting this both here and to Primal Core's issue page, since it's their recipes not working, but it seems to trigger with a change to Mystical World's config.

Mod list:
forge-14.23.5.2838
Just Enough Items (JEI)- 1.12.2-4.15.0.289
Mystic World 1.12.2-1.4.2
MysticlLib 1.12.2-1.3.3
Primal Core 1.12.2-0.6.105

issue on Primal Core's page:
An-Sar/PrimalCore#345 (comment)

From what I can see, this is most likely an issue with Mystical World creating recipes for individual metals (nuggets <-> ingots <-> blocks) with checking if the global values for these recipes are enabled, not whether the specific metal itself is actually enabled... this would result in potentially a recipe conflict in the Vanilla registries.

Not 100% sure but I'll dig into it -- this code is quite messy and should really be extrapolated out into JSON.