I have a ROKU 4800X plugged in to a Hisense U8G. Regardless of the app I use (Netflix/Disney+/HBOMAX/Amazon Prime) it will only output HDR and Dolby Vision content in RBG 8 bit.
The ROKU detects 4K Dolby vision. I have tried both 4:2:2 and 4:2:0. My Panasonic 4k UHD player correctly outputs 10 bit in YUV420 and other color formats so I know its not the TV