I have this issue too and would love some advice.
In my case, I see it on my display which is a vizio m43-c1 uhd panel. I'm using the HDMI 5 input (only one that's 4k 60hz capable) and when I set refresh rate to 60 hz, I find that in nvidia control panel color depth is set to 'limited' and is greyed out. I understand that the panel can't do 4:4:4 sub sampling and in fact seems to be limited to 4:4:0 at 4k 60hz, but why this would cause color banding or why color depth would be locked to 'limited' doesn't make sense to me. I expect mildly blurry text from the chroma subsampling, which I see but it's not bad compared to how it looks at 30 hz 4:4:4. I can't see that using 8 vs 10 bit color in nvidia control panel changes anything in-game.
GPU is a 980ti and running HDMI to the display. Card is spec'd as HDMI 2.0 capable.
The banding is very apparent especially when the ED logo fades in at launch.
Any tips/info appreciated.