Turn it off and on again?
Tried re-installing the app?
Was just going to post about experiencing this today. Did you find a fix? Turning the TV on and off again didn't help.
Get the same issue when casting directly to the built in Chromecast receiver.
try switching your HDMI port if you can?
If it's built in, try changing resolution, otherwise sol.
Just go to you Android TV settings >Apps>YouTube>uninstall updates. After uninstalling the update disable auto update in the PlayStore.
Looks like another quality job from the youtube app team