I have experienced the same problem and have found temporary workaround that deal with this until ROKU issue a fix.
The first one i found useful was to change the video setting to 4K HDR 30 HZ instead of 60 HZ.
The second thing that worked for me was to change the advance setting from 4.2.2 to 4.2.0
Either one of these changes fixes the problem immediately for me. Hope it helps you as well