Custom superflat generation seem to be inconsistent when putting in structures/decorators with the new world height.

Structures such as geodes, mineshafts are generating only above y=0.
Strongholds seem to be not generating at all.
Lava lake features not generating below y 0.

Most decorators show skewed and inconsistent generation vs default overworld gen:
Tuff and gravel patches not generating below y 0
Redstone, lapis lazuli, iron ores not generating their usual heights below y 0
Diamond ore generating abnormally above y 16

Other biome-specific decoration has not fully been tested.
For windswept hill (and likely any mountain biome)-related generation, infested blocks are not generating below y 0.

How to spot/reproduce:
Generate a custom superflat world with stone or deepslate for ores and other things to generate in. I suggest filling in the range of the ore distribution.
Reveal ores and check for structures.

Superflat Presets:
Tunnelers' Dream for best observed results.
Overworld, Desert, and Snowy Kingdom will show the lower decorator inconsistency.

For the images I generated a custom preset (no datapacks; modifying Tunnelers' Dream)

minecraft:bedrock,64*minecraft:deepslate,230*minecraft:stone,5*minecraft:dirt,minecraft:grass_block;minecraft:windswept_hills