I guess the following is how the preferences work ?
2160p30 and below : RGB 12-bit, YCbCr 4:2:2 12-bit, RGB 10-bit, RGB 8-bit
2160p50 and above : YCbCr 4:2:2 12-bit, RGB 8-bit.
(There are no 4:4:4/RGB modes at >8 bit supported in 2160p50 and above in HDMI 2.0, and 4:2:2 YCbCr is supported at 12-bit only)
4:2:2 12-bit YCbCr at 2160p50/60 is often only supported by some (not all) HDMI inputs on some TVs (like my Sony XF9005 - which won't support this format on HDMI 1 and 4, only 2 and 3) and has to be enabled in TV menus. Similarly my Denon AVR has to have support for 4:2:2 12-bit enabled in its menus too. It's often called 'Enhanced HDMI' or similar. It also requires higher performance HDMI cables to work reliably as the bandwidth is pushing the limits of the HDMI 2.0a spec.