So this is an annoying world gen bug that makes nether look ugly. It exists only in Java edition but not on bedrock
Basicaly vegetation in the crimson/warped forest in the nether can only spawn on the upper most layer of the biome and every layer under it will not have any vegetation spawning on it
This can be seen on litearly any world but here's the seed:
8715353494941378358
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However, we are already tracking this issue, and this report is a Duplicate of MC-171363, which is a report that has not been resolved yet.
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