This is a very old problem that still presents today with 4.2 alpha. The broken depth channel when rendering with volumetric fog or cloud, has completely blocked blender in serious VFX works.
It seems this problem is unfixable because it has been here for 5-6 years since the first issue was created.
Some one may say you can use EEVEE to render z depth, but I had to say that eevee’s depth channel is too blocky to use, in another sense it’s also broken.