I had the same problem. I went into setting, system, power and found a setting that automatically turns off the ROKU Ultra power after 20 minutes. I turned that automatic shut down OFF. Everything has been fine since.
I found that the quickest and easiest fix was just to power it down and back up again, or use a restart in the settings menu. Clearly there was no improvement from swapping out hdmi cables. The problem lies in the Roku hdmi connector, Roku software, or Roku internal hardware. All I can say is that mine pretty much disappeared after about a year, which I assumed was due to a Roku software update, but that’s probably not correct since so many other folks are having the same problem. I don’t think either that Roku ever fessed up as to what caused it.
I had to resort to a hardware approach. I took an open-end usb cable and attached the power leads to a relay. The relay contacts break into an extension cord that feeds the ROKU power adapter. This starts the ROKU from scratch instead of already being powered up and running. All of the tv HDMI is ready before the ROKU HDMI starts to communicate, which appears to have been the problem. The ROKU folks could probably just put in some sort of delay to the HDMI port to allow the tv to "boot".
I forgot to mention that the usb cable that feeds my relay gets plugged into the tv usb port. That way, when the tv is turned on, it switches on the roku wall wart instead of it being powered on all the time.