Players respawn inside the bottom bedrock layers when the world spawn point is set in the Nether

Hi!
Thank you for your report! 

• Does this issue occur only on this specific seed and location?

• If yes, can you share your world’s seed?

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After I created another world for testing

• No restrictions on world seeds

Can confirm

Additional Context cloned from MC-303391:

From Me:
@OP and I were both tracking this problem:

If the Biome / Level in question does not have Skylight the same problem happens; such as if World Spawn is in the Nether; or the Level is set to not have Skylight access.

A quick fix would be to forceload the spawn chunk (legacy behavior); or Overwrite the World Spawn selector to make it equal to the correct world spawn.

From Bread:
More context, from searching in code i tracked down to PlayerSpawnFinder::findSpawn(ServerLevel, BlockPos)
the very first if condition checks if the biome has skylight, or if the server gamemode is not adventure and if that is true follows through using a method that will issue chunk loading tickets, but the secondary path instantly resolves the spawn location, and the chunks may not be loaded yet allowing this bug to happen

Additional context:
The problem happens when the Chunk is not pre-loaded during the spawn attempt because it cannot resolve that the Spawn Location has solid ground using the heightmap calculations; since the Chunk is not yet loaded; so it sets the Y level to 0 then adds 1 .