when you generate a buffet world with the cave world type it generates the bonus chest above the top bedrock

To reproduce:
1. in the create new world menu click "More World Options"
2. Click "World Type" 4 times until it says "Buffet"
3. Turn on cheats (for later)
4. Turn on "Bonus Chest"
5. under "World Type" Click "Customize"
6. click "World Generator" once
7. Choose a biome (I used plains)
8. Load world and go to spectator and fly up