Crimson and Warped forest can spawn on top of the bedrock in the nether, also there can also spawn mushrooms.
MC-185959
Crimson and Warped forest spawns on top of the bedrock in the nether
Resolved
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Mushrooms are intentional, see MC-1381 - as for the forest, I cannot reproduce that - if you used the world in previous snapshots, it may have been a bug there.