Wide Color Space and Low Bit Depth often results in Quantization Error (Banding).
Use ProPhoto with 16 bit and AdobeRGB with 8bit.
JPEG compression on very subtle gradients, even at 12, could worsen the problem a little more.
Thanks for your reply Doran.
The problem happened using ACR 6.0, ProPhoto, 16 bit. But it was "solved" (or improved) by using AdobeRGB in 16 bit.
And the problem did not happen using ProPhoto in 16 bit when using ACR 5.6.
I don't really want a "work-around".
I would like to be able to:
1) Have the interface accurately represent what I will get in the final file.
2) Be able to work in ProPhoto 16bit and get output that does not have banding and blotching.
Do you mind sharing the raw file? Along with adjustments (exposure, sharpening, NR etc) that cause the banding.
Strongly do not recommend using ProPhoto and JPEGs. I have cases (i.e., test files, with blue skies, and other smooth areas) where you will get color blotchy artifacts if you save as a JPEG with ProPhoto, regardless of the raw conversion software used (or version).
My recommendation: if you want ProPhoto, save as TIFF. If you don't plan to do much editing afterwards, even 8-bit TIFF (either with LZW or ZIP compression) is fine. If you want JPEG, then use Adobe RGB.