Steps to Reproduce:

1. Create a superflat world.

Observed Result:

1 - Random chunks were generated with regular world terrain, not flat.

This seems to be a very rare bug. (These kind, I hate, since it can be a number of combinations). I had the render distance set to 32 chunks, but future superflat maps with 32 chunks loaded (same and different seed) didn't produce the bug. I also tried recreating the world with the same name (I've seen this happen before), and creating a custom generation world prior to the superflat world, but also didn't reproduce the glitch.