The way the Deep Dark biome generates right now, the entirety of a selection of the "mountain" biomes become Deep Dark once you hit the deepslate Y levels. This was supposed to happen in the same way that Lush Caves and Dripstone Caves generate in 3D blobs, not just "If biome == Stony Peaks || Jagged Peaks && y < 0 set biome = Deep Dark", which is what is happening now. The knock-on effect here is broken rare ore generation for mountain biomes. (Similar to the reported issue affecting Lush Caves / Dripstone Caves, but worse as Deep Dark biomes have no ore generation overrides the way the other two cave biomes do.)

Known broken:

Stony Peaks

Jagged Peaks

Known working:

Grove

(I will update the above list as I can validate the various biomes.)

Recommendation:

Either:

1. Fix Deep Dark generation so that it is not simply a switch for a given biome at a given Y level.
2. To address the knock-on effect, allow rare ore generation for the original biome that the Deep Dark biome replaced.