I'm fairly sure Coverage Adjust in the Cloud Shader is "working properly" it's just a misunderstanding of how it works. It is adjusting the "offset", i.e. it's adding to the value coming into the Density Shader input. The value range is from 0 to 1, so any positive offset will inherently make the "baseline" (the "darks") greater than 0, which gives a light overall coverage, i.e. this ugly, flat, grey sky. The reason it works better/differently in the Density Fractal noise function is that the fractal generates both positive and negative values, so offsetting it there - before it gets to the cloud layer - produces the more likely intended result of expanding the "light" areas since they are being lifted out of "negativeness".
- Oshyan