This bug was tested in 18w05a since 18w06a does not support customized worlds, which are required to see this for the most part.
This bug is extremely similar to MC-3347.

It does not seem to be possible for caves to generate at sufficiently high layers, even where there is stone present to allow caves to exist. The highest point caves seem to be capable of generating at is y=168 or so. Other underground structures such as ravines and abandoned mineshafts, but excluding water and lava ponds, also appear to be affected. This can be seen by generating a world using the preset attached and exploring through the ground via spectator mode; caves never seem to be able to break the surface at y=255.

Attached is a screenshot of the highest point of any cave I have seen so far, along with its seed.