So I have a Ultra also. Awhile ago I was getting the HDMI cords loose error and I would unplug and replug them but that didn’t work, but when I un plugged the power cord and plugged it back in it worked.
now whenever I start to watch the roku I will get a small viewing screen in the corner of my tv instead of the full screen. If I un plug the power cord and plug it back it goes back to normal but I have to do this every time I try to use the Roku.
See if you have a "power saving" option under Settings->Power and disable it if you do. You may also be able to restore the screen by switching your TV's input/source away from the Roku and then back.