The ore spawn effect was created before deepslate was added, and now it just generates any overworld-style ores in stone and none in deepslate.

If possible, it should generate stone ores in stone and deepslate ores in deepslate, but there might not be a way to differentiate them through tags.

Either way, generating deepslate (& other mods' dimensions/stone types) ores in stone is a bit silly.