When I make a custom superflat preset in the 1.8 snapshots of Minecraft, the default superflat world of 3 dirt/grass and one bedrock overrides it.

Steps to trigger the bug:
1. Open Minecraft
2. Click the customise drop down of superflat world.
3. Enter a custom preset.
4. Click done.
5. Enter the world and you will see that the world is just the default superflat terrain.