I was in touch with him by PM. This is because he was using a recent driver (39x.x). As I reported yesterday, each 39x version broke something new, at least here. The best driver to use with a JVC is 385.28. With this, you can get correct levels in RGB Full, 12bits in SDR and HDR, and he will only get the magenta bug (which can be fixed, as I told him, by disabling "Sending HDR metadata) at 4K60p.
With a recent driver, he can solve things if he disables "Send HDR Metadata" and uses 8bits in the GPU. But that's not an option for me at this stage (I need the metadata for the Vertex), so I went back to 385.28 for now.
The main downside of 385.28 is 3D (1 frame drop repeat every 3mn vs every 13mn with 397.93), and very minor banding in HDR passthrough (not sure when that was introduced by nVidia, but it's still here with 39x.x). You can solve most of the banding in passthrough using 9bits instead of 10bits dithering in MadVR, unfortunately that's not possible with the latest MadVR build, so you have to revert to the previous one.
The JVCs do have some quirks, but they can be used fine if you know how to drive them
