Versions:
Minecraft: 1.12.2
Forge: 14.3.5.2854
Crafttweaker: 4.1.20.586
Contenttweaker: 4.9.1
BASE: 3.13.0
Optifine: HD-U-F5
Shader pack: SEUS-renewed 1.0.1

Custom ores added by contenttweaker do not render correctly, maybe has to do with the texture of the ore being added on top of the existing block texture and that conflicts with the normal maps?

It seems that you can see the cave lighting underneath:

Here is another angle, where you can see the undersides of blocks:

What is interesting is normal blocks (ex: blocks of metal) render just fine:

The custom ores rendering is based on transparent, sadly.