Minecraft 18w19a, Minecraft 18w19b, Minecraft 18w20a, Minecraft 18w20c, Minecraft 18w21a, Minecraft 18w21b, Minecraft 18w22a
Certain world configurations cause the player to spawn at y=5, even if there are solid blocks in the way. This seems to happen only upon first entering the world.
I'm not exactly sure why this is the case or which configuration settings cause this, but it seems to affect superflat worlds where the top layer is a block that doesn't "floor the world's biome" (ex. grass in plains, sand in deserts, mycelium in mushroom biomes).
This can be most easily observed by using the "redstone ready" superflat preset.
1. Create a new superflat world with the preset "redstone ready"
2. Notice that the player spawns inside the sandstone. This does not seem to happen for "classic flat" worlds or normal worlds.