After exploring underground generation in Spectator mode for a while, I noticed copper ore veins, when intersecting with ravines in generation, will generate floating granite, but with no copper ore. It is pretty clear that there is supposed to be an ore vein there. If you dig into the walls of the ravine where the ore vein is, you'll see the copper ore generating there like usual. So, only where the ravine is, is where no copper ore seems to generate.

Not sure if this affects iron ore veins also, haven't found this happening with them yet.

Instance:

Seed: 5946919130809614284

Coordinates: -2817.77 29.60 1201.49