I created a world in 1.6.4, closed it, and then loaded it up again in 14w11a. All of the chunks spawned separately (meaning that there were flat walls and half-trees). There was even a jungle temple floating in the middle of flat water.

Put the summary of the bug you're having here

I expected the world to load normally.

The world spawned in separated chunks

1. Load a world in 1.6.4, and don't move. Just stay where you spawned
2. Load up 14w11a, and go into the world.
3. Fly around and observe the environment.