The bug
When you spawn in a some biomes a warning is printed in the Game Output. This can be easily reproduced by creating a "Redstone Ready" superflat world or by creating a desert world using the "Customized" generation.
[Server thread/WARN]: Unable to find spawn biomeHowever there is no crash or something similar.
Affected biomes
Incomplete
desert
snowy kingdom
void
The reason
The following is based on decompiled version of Minecraft 1.8 using MCP. All method and class names are the names used in the decompiled version.
The reason why this happens is that for the superflat generation the net.minecraft.world.biome.WorldChunkManagerHell is used. The method public BlockPos findBiomePosition(int x, int z, int range, List biomes, Random random) which gets the spawning position returns for the net.minecraft.world.biome.WorldChunkManagerHell class null if the biome used for generation is not in the private List biomesToSpawnIn. This makes however no sense as a superflat uses only this biome. This means the private void createSpawnPosition(WorldSettings p_73052_1_) method of the net.minecraft.world.WorldServer class should only try to get a spawn position if the generation type is not superflat.
Linked issues
Comments 7
This may have extended to most or all biomes in 20w28a, just experienced it in a Birch Forest with Large Biomes. More testing required.
Confirmed.