I had a Roku 3 for many years. It was a fantastic product. I used it for 4 years without issues. Then, some months ago, I decided to upgrade to a Roku Ultra. This was a mistake. On the very first day, the screen would go blank for around 1 to 2 seconds, 1 to 2 times per hour. I never had this problem with the Roku 3. Not once.
The obvious thing to do is to call Roku support, which I did. Despite the fact that I never had any problem with the Roku 3, they tried to convince me that there must be a problem with my HDMI cable. So, I bought a new one. Screen was still blanking. Then they tried to convince me that there was something wrong with my TV, which I knew was ridiculous. Then, they thought that there my be something wrong with my specific unit, so they sent me a replacement, which still had the blanking problem.
I stayed on top of my tickets with Roku support, and had to continually push to get status updates about my tickets. Multiple times, after escalations, multiple people at Roku support promised to follow up with me, and never did! I was going out of my way to try to help them solve the problem, which was obviously the operating system. Perhaps there is just a bug in their OS that does not like my TV. I have no idea. But, I offered on multiple occasions to help them try and debug the problem. They promised to follow up with me more times, and never did.
After the many failed promises to follow up with me, that were never fulfilled, and with waiting for status updates, the time period in which I could get a refund from Best Buy lapsed. I honestly believed that Roku would want to find out why their device was intermittently ceasing to send a signal to my TV. I thought that Roku would care enough about customers to follow up when they promised to do so. But, none of these turned out to be the case. Instead, what I got was the cold shoulder from Roku, despite being a loyal customer and trying to help. And despite it being their fault that I was not able to return the device, due to waiting on their follow ups, they refused to issue me a refund.
Long story short, I bought another device from a competitor. And guess what ... IT NEVER ONCE MADE MY SCREEN GO BLANK. It works just fine with my TV and HDMI cable.
So long Roku! I enjoyed the Roku 3, but your current OS simply does not work well, and you did nothing to try to give me a device that I could actually use. You cost me more than $100 for a device that does not work and refused to refund my money.
Even though I don't use a 4K TV, I upgraded my 3 to an Ultra recently simply because of the MLB.TV glitch discussed elsewhere on this forum that impacts the 3 this season. For all the other channels I use, the 3 still works just as well and I wouldn't have updated otherwise. Knock wood, the Ultra has been problem-free for me.
Based on the information you gave, my best guess is that this is a problem with the HDCP 2.x support on the Roku Ultra performing a periodic handshake that the TV isn't dealing with well. I know you have been rubbed raw by how Roku support has handled the case and been pointing fingers, but I would still recommend contacting the manufacturer of the TV to see if there is a firmware update to the TV available. The reason I suggest this is the Roku may be blindly probing the TV to see if the TV will accept a HDCP v2.x packet which a TV that only has HDCP v1.x support should just ignore. It might be there is a bug in the specific model of TV which causes it to blank for 2 seconds when a correct HDCP implementation should not be an issue. If my theory is correct, this would need to be resolved by an update on the TV rather than it being a Roku OS issue.
Out of curiosity, did Roku support ever direct you to the HDMI Secret Screen by doing Home x5, Down, Left, Up x3?
Would you be willing to post the results from that screen? What does it say under MFG, HDCP status and HDCP version?
Also, I am not trying to attack your logic that moving to a competitor device works fine indicates it was never a TV/cable issue. However, if the competitor product doesn't support HDCP v2 because it isn't capable of 4k resolution, then there might still be an issue a TV firmware update could resolve for use with the Roku Ultra.