when you get far enough up, you can see the entire planet, but when it renders, parts far away are invisible when you look at them. this happens on every planet, and looking up or down makes the effect not as noticeable.
attached are 3 screenshots of the corner of the planet not rendering in on earth, the moon, and mars
This happens everywhere because of how Minecraft Java renders things. For example, Bedrock Edition has a more rounded edge and because of that you don't notice it as much.
when you get far enough up, you can see the entire planet, but when it renders, parts far away are invisible when you look at them. this happens on every planet, and looking up or down makes the effect not as noticeable. attached are 3 screenshots of the corner of the planet not rendering in on earth, the moon, and mars