Since Noel is back with us, I thought this header might catch his attention. He was the one who first reported it six or seven years ago.
The reason I'm bringing this up now, is that someone in the Lightroom forum had this problem just yesterday (he was sending ProPhoto files to Photoshop and couldn't understand the cyan cast in his dark grays). There's no doubt it's the same issue.
This made me dig up my old ProPhoto test gradient file. I don't use ProPhoto all that much, so I haven't really looked for it in a while. To recap, this is an example of what it could look like (this is an old screenshot that I've kept). Open and view against a darkish background:
But when I opened the test gradient now, there was no trace of it, zero. The gradient is smooth as silk. That surprised me, because while the effect can vary, it always seemed to be there in some form.
Technically, this is an inaccuracy in the conversion from ProPhoto into the monitor profile. This conversion is performed by OpenGL in the GPU when you have the PS preference set to Normal and Advanced modes. In Basic mode it's shifted back to the CPU, which is more accurate so the problem disappears. And you only see it in ProPhoto files because ProPhoto is very compressed in the shadow values compared to other color spaces. The stratospheric gamut has a price, and that's it. With this compression, inaccuracies get amplified.
So what has changed on my system over the last 12 to 24 months? Not much. I still use Eizo Colornavigator as I did then, still producing similar matrix monitor profiles. But one thing has changed - going from GeForce to Quadro, and with that, 10 bits per ch output.
So Noel, if you're reading this - I understand you have Quadro GPU too, and running displays at 10 bit. Do you still see it?
Anyone else?
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So if I understand you correctly, you are asking why you don't have a problem? 
So this is the same issue as when I export from Lr to Ps via Edit In?
I'll try one in Basic mode and update the previous post.
Update, I can't seem to edit the previous post so here's with Photoshop in Basic drawing Mode
And here are the 16bit files. I always work in 16bit but since yours wasn't I wanted to test with the same settings.
You shouldn't get this in Basic mode. You obviously know that changing drawing mode requires relaunching Photoshop, so I will not remind you about that 
Is the banding cyan colored (that's the smoking gun), or just regular banding? See my original screenshot at the top.
OK. I suppose everyone has more than enough to chew on these days, between color blend bug, constrained scaling and file associations, so I won't stress this too much now. I was just curious. Also, I need to mind my job inbetween...
See whether Use Graphics Processor is enabled or disabled in your Performance Prefs, and what Drawing Mode you're set to use. As I recall setting the latter to Basic moved the color-management tasks into the CPU and that worked around the problem.
I just reproduced the problem here on 20.0.0, 30 bit color, with the nVidia 391.89 driver. For a gray graident in a ProPhoto RGB document going from levels 12 to about 90 I see concentric rings of cyan and gray. They look nice and smooth with the 30 bit color, but the color inaccuracy is definitely there. Here's a saturation-enhanced screen grab (which necessarily reduces the image to 8 bit color)...
I know you've recently updated your nVidia display driver, Dag, to 4xx.something. I've been meaning to do that myself. I will do so shortly and report back.
-Noel
I cannot believe we're the only ones noticing this issue, this is in B&W....or is it color?
I've been using Basic drawing mode for ages now because of this and even bought a new GPU a week ago to solve this.
Alright I'll do some more testing to see if I can make the switch to AdobeRGB working space (advanced)
Hi Dag,
Oops, got nVidia 411.81 here. Apparently released today. I'm still on Windows 8.1 by the way. The problem remains.
Whatever you've got going on, treasure it. In all seriousness, it's good that you've picked this issue back up. Maybe the difference will lead to a discovery.
-Noel
It looks compressed but isn't this because the image is dark and has more a lot more information in the ProPhoto space?
As you already guessed though I'm not an expert in this, by far, I wish though so I could solve this problem. I certainly know my way around Lr and Photoshop where image editing is concerned but this stuff makes my head spin.
Adobe RGB vs ProPhoto RGB
Now I'm deciding between ProPhoto RGB with Basic or Adobe RGB with Advanced. This is with RAW files, final export to sRGB or aRGB, depends on the library I submit to. Do you think I won't miss much by using Adobe RGB? If I was shooting JPG then sure it would make no sense to work in ProPhoto although.... I've read you can get smoother gradients (or other edits) when converting to 16bit and then save back to 8bit.
Oh and WELCOME BACK NOEL!!!
https://forums.adobe.com/people/D+Fosse wrote
You shouldn't get this in Basic mode. You obviously know that changing drawing mode requires relaunching Photoshop, so I will not remind you about that
Embarrassed time. I was thinking it was the same as GPU on off and just needed a new document start. I'll retest later tonight 
Dave
By the way, do you have the Use native operating system GPU acceleration setting available and checked in the Advanced Graphics Processor Settings panel. It's grayed out for me; I imagine it's only available for the very latest OS versions.
-Noel
Indeed, Web (converted to sRGB I suppose) and Ps version side by side
Btw are we in a private room listening to our own echos or are Adobe engineers actually looking into these threads?
We really need to get our hands on one of those engineers asap, I'll bring the chair, you bring the rope, D Fosse can ask the questions!
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