Apple TV 4k mini on screen
How do you fix the apple tv screen on my tv?
Apple TV 4K, tvOS 15
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Typically that is due to a picture setting on your TV, not the Apple TV itself. Refer to your manual if unsure on how to change that.
If you did want to cross check on the Apple TV then you can back out into the main menu to go into it's settings, then video and audio and check its video output format.