Tinkers' Construct uses the first nugget oredict entry as the result of the nugget cast for any given material. We register our oreberries before our own nuggets, so unless another mod registers a nugget before we do, it will end up getting cast into our oreberries.

Kinda-sorta fixed in 0.3.2 by 3131337. I'd still like to do some sort of actual blacklisting like https://github.com/SlimeKnights/TinkersConstruct/blob/1.7.10/src/main/java/tconstruct/smeltery/TinkerSmeltery.java#L940-L948, but the only blacklisting API that's exposed in 1.12 is to prevent melting, not casting.

Are you using Unidict?
There is also a option to add preferred mod IDs in TiC's config file.