Never mind. setting it to 4:2:2 didn't really solve it. I get screen flashes with it as well.
Temporary solution is to unplug HDMI cable for a minute or so then plug it back in. As soon as I turn off the system or switch the receiver to another input to watch something else it usually comes right back with the flashing screen.
I downgraded the firmware back to 7.6 and it's working fine right now. Dreading it updating to 7.7 again or I'll be back to unplugging and plugging in the HDMI cable every time I want to watch the Roku.
4K HDR TV: Samsung UN60KS8000
4K HDR Receiver: Yamaha RX-V381
4K HDR blu ray: Samsung UBD-K8500
4K HDR streamer: Roku Ultra 4640 (2016 model)
4K HDR Gaming: Xbox One X
4K HDR DVR: Directv Genie HS17-500 w/ C61K mini client
Cables: Amazon Basics 18Gbps HDMI