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
.