World generation buggy!!

World Generation Error!

What I expected to happen was...:
I expect to have a super flat land when making a new flat land world

What actually happened was...:
i spawn on a default chunk and there is also a flatland chunk side-by-side

Steps to Reproduce:
1. Open singleplayer
2. Click "Create New World"
3. Click "More World Options..."
4. Change "World Type" to "Superflat"
5. Click "Enter"