More info: The world got corrupted a week later. (Not sure of this relates.)
Funny enough this happend to me when making a Minecraft map.
Except the solid color on top was always black in a certain area about the same size as yours.
I tried everything from replacing the grass to redownloading Minecraft.
The map was ruined so I quit making it.
Now the world is corrupted about a month later.
Hope this helps 🙂.
Same issue happend to me!