Any flat world presets have minecraft:ocean biome type at Y=320, instead of the biome type set in their presets.
Comments 3
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My supposition:
The height limit of the world is 320, but actually Y=320 is not involved in the highest subchunk (where Y is between 319 and 304). And the biome type is stored in the subchunks, so when you were trying to make out the biome type of the location outside level range, the default is minecraft:ocean (id: 0).
In that case, I appeal the void biome back. And apply it for the location outside level range.