I wonder If Java Edition has the same issue as Bedrock edition (https://bugs.mojang.com/browse/MCPE-121708, in which they appear inconsistent before caves and cliffs.

(Copied From Gruntastic's report above)

1. Create a new world with experimental features enabled
2. Run the following command as you move around the world:

   /fill ~-1 -63 ~-1 ~1 ~ ~1 air

1. Caves will only generate properly below layer 0 and less commonly above layer 0.
   1. The caves will also not generate through any custom blocks
2. Expected Results:

• • The cave carver feature works

1. Actual Results:

• • Caves don't generate