Color Picker Banding in ProPhoto RGB

If GPU Basic doesn't clear it, it's not the OpenGL issue.

But then sRGB as monitor profile _should_, because then it has to be a defective monitor profile (or actually Adobe RGB, not sRGB, since it's wide gamut).

Did you relaunch Photoshop? You need to in both cases.

If you're using the BenQ software, it's pretty buggy and unreliable. There have been several threads about it.

This is a problem with the monitor profile. Are you using a calibrator to make your profiles? If you do, rerun it, and make sure it's set to make version 2 and matrix-based profiles (this is usually the default in most calibrators). Version 4 and/or table-based can cause problems.

If you don't have a calibrator, use sRGB IEC61966-2.1 until you do. You probably received a bad profile through Windows Update. sRGB is not entirely accurate, but better than a broken profile. Relaunch PS when done.

It's not uncommon that problems mostly show up with ProPhoto files. It's such a huge color space that small inaccuracies get amplified.

There is, for instance, a known problem with GPU / OpenGL handling of ProPhoto files, visible with GPU set to Normal/Advanced mode. The most common symptom is cyan/red color banding in the shadows (the problem is in the red channel). It does not happen with Adobe RGB or sRGB.

To test if that could be your issue, make a dark circular gradient. It may look like this:

Nevermind the out of gamut warning. That just refers to your working CMYK, not of any relevance here.