I have a Roku soundbar attached to a TCL C Series/C807 2017 65" TV. All software is up-to-date, and I use the sound bar to power-on everything and use the Roku interface. The TV is basically a monitor at this point, but I've attempted to set the resolution 3840 x 2160 at every setting I can find on both devices.
Unfortunately, about 50% of the time, when I power it on, it'll display the Roku UI in a low-resolution 4:3 aspect ratio (you can't even see all the entire UI). Powering off and back on usually fixes the issue, but it's been going on for a little over a month now...
I've set the TV to 3840 x 2160. The smart soundbar is set to auto for display as setting to 3840 x 2160 broke video playback for some titles (audio but no video).
I've also tried "Disable Automatic Recovery". I managed to take a photo of the strange 4:3 low resolution that is randomly occurring. Turning everything off and back on again appears to fix the issue temporarily?