The bug
In a Floating Islands world, the bonus chest can spawn at Y=0.
To reproduce
Create a new world with seed 219852490231203955, enable the bonus chest, and set world type to "Floating Islands". Customize the world and select "Plains" as the biome. The bonus chest will generate at Y=0.
Can confirm. This does not happen every time. I was able to confirm it in a Plains buffet world with the seed
219852490231203955.