I went to 300000000 Blocks m to end adventure then I found THIS.

Nothing is truly infinite. The worlds have a limit of 9 million blocks.
It's not a bug (you'll never be able to do this in survival, so do not bother yourself so much ...)

+Not A Real Name Yep, this works as intended. When you go super far from your initial spawn, you will see another world, the "far lands". It's not a bug.