The bug
Upon entering the (freshly created) Debug World, it spawns you at y=2, but the blocks are at y=70. If you die, you still spawn at y=2.
This first occurred with snapshot 18w19a.
⚠️ Side note: Entering a world with Void World preset will spawn you at y=0 (MC-130523) even if you entered a higher air block number.
Can confirm for 1.13pre3.