That's weird. Don't see the same behavior in my Mi Box. What version of Android TV/Youtube app are you running? It looks like the YouTube app is triggering a notification but something is not quite right. I think they might bring some kind of support for background audio-only playback since they integrated YouTube Music in the same app, this might be a half baked, broken implementation of that?
Have you tried logging in?
Im finally able to disable this popup by going into system apps ( android core services) and turning off notifications button.
What box is this?