new nether forest trees generating above bedrock

above the nether's top bedrock layer, the new nether forest trees generated

I expected to find the area above the top bedrock in the nether completely flat. When I went to the upper area in the nether, I found crimson stems generated on top of the bedrock

Steps to reproduce
1. in nether tp above bedrock
2. use /locatebiome minecraft:crimson_forest, or /locatebiome minecraft:warped_forest
3. teleport to the nearest biome