With an external playback device connected to your TV, your channel is only playing on a portion of the screen instead of the entire screen.
When a playback device that is connected to a TV screen is already running when the TV turns on, the TV might not always be able to adopt the resolution of the connected playback device.
Make sure the playback device turns on after the TV screen turns on.
If you always leave the playback device running permanently and only schedule the TV to turn itself off and on, then you could leverage the restart daily at feature of a registered playback device provided that this device runs either ChromeOS or Android. Simply schedule the player to be restarted daily a couple of minutes after the TV is turned on. That way the TV will detect a new signal on the HDMI input and it will switch to the correct resolution.
An different solution is to either always turn both the TV and playback device off and on at the same time or never automatically turn any of them off. The latter is obviously the least energy efficient.