God bless you! I was going through the Hue Sync Box. My Roku Ultra could tell it was connected to Dolby Vision capable TV, but Sunc box was not... once I read your post, I bypassed Sync box, forced 4K HDR, and then reconnected to Sync box. BOOM!
Now why this started happening randomly after 6 months, who knows.
The fix of switching display from Auto to actual resolution has worked here so far. I had opened a return ticket. I'll wait for a few days. I still have a 1080p TV so that's what I selected. It was losing signal quite often on auto. Several hours since I switched it and way better. Thank!
Here are the steps we would recommend to fix the issue you're experiencing:
The video cable is not connected properly or the cable is faulty
Check that the video cable is firmly connected to the back of your Roku player and to the HDMI or composite connector on the TV. If you have a Roku Streaming Stick, then make sure it is firmly connected to the HDMI connector on your TV.
If you can, check to see if you have a faulty cable by connecting your Roku player with a different cable.
There is a problem with the TV
If you can, try connecting the Roku player to a different input on the TV to see if the problem is with the connector on the TV.
If you can, try connecting the Roku player to a different television to see if the problem is with your Roku player or the TV.
If all else fails…
Disconnect and reconnect all cables including the power cable.
Roku’s are not terribly expensive compared to the level of technology you are getting. My guess, they don’t have a lot of full time people to answer questions. They create a user forum like this so we can assist one another. If you will post the steps you have tried; maybe we can help you get things sorted.