Certain world configurations force the player to spawn at x=8 z=8

The bug

Certain world configurations seem to spawn the player at 8 ~ 8. This seems to happen regardless of y-level, and, similar to how MC-129222 was, 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.

Update: As of 21w06a this also affects the y level, with the player spawning 64 blocks higher than they should.

How to reproduce

1. Create a new superflat world with the preset "redstone ready"

2. Notice that the player spawns at 8 ~ 8. This does not seem to happen for "classic flat" worlds or normal worlds.

A demonstration can be seen here.