I'm still very confused if it's just the game being very bad at generating stuff or if it's actually a bug, but if it isn't It would be still cool if a fix was possible in some kind.
I'm making a dimension mods needing nether generation and one my biome have a top layer of block x and a lower layer of block y, then the normal dimension block. I've sat that an ore should very frequently spawn in this biome, dimension and replacing the block x (top layer) with (amount of ore groups per chunk of 48). Right now, I've been searching since 30 minutes and I can't find any of this block on my top layer or it's waaay too rare. That's a huge problem as I need this type of mechanic.