How to make the ore completely covered with blocks during generation?

There are two ways to do this to my knowledge:

You could make a generation condition in the actual block element to test if all faces (up down north south east west) are all not air, if that's the case, only then would it place the block.

Or...

You could instead make the ore generated by a feature element instead of via the generation tab in the block element. There's a template for generating buried lapis that you could use and just replace the lapis with your custom ore, and it will generate buried.