Minecraft end of flatlands?

While exploring Minecraft in my flat country, I came across these "walls" behind which there seems to be a normal world. (see picture). I haven't found anything about it on the Internet, except that you have to search a long time for the end of the world, which I didn't have to, it was only about 700 blocks away from my spawn point. Does anyone know something about it or is that normal in a flat world? Version 1.16.4

You can find the world barrier from the center point (coordinates 0,0,0) 30 million blocks in one direction.

Everything before that (i.e. X or Z coordinates between 0 and 29.9 million) are errors in the world generation.