In the recent snapshots, terrain-dependent sub-biomes (like swamp hills, taiga hills, etc.) were removed in favor of terrain and biomes being independent from each other, with the same thing happening for the mushroom fields biome (with mushroom field shore biome being removed). In spite of this, mushroom fields are not affected by the terrain changes despite this being the case with every other land biome, and with it's sub-biome being removed. For clarification, the bug is that the mushroom fields biome is not effected by the new terrain generator, not that the mushroom field shore biome isn't generating, as that was marked as WAI.

Steps to Reproduce:
  Step 1: Create a new world
  Step 2: /locatebiome minecraft:mushroom_fields
  Step 3: Look around and notice the terrain being quite flat and low compared to the average terrain you'd normally find, even when the f3 screen says stuff like "Peak" or "High"

  Step 4: Repeat steps and notice the same flat terrain with every mushroom fields biome

Observed Results:
Every mushroom fields biome has the same flat terrain reminiscent of 1.17 terrain generation even when the f3 screen makes it seem like there should be a giant mountain.

Expected Results:
The mushroom fields biome generates in the same wacky and unique way every other land biome generates.

Notes:
While this might look like it is WAI as the developers have stated that the ocean biomes will be staying the same, and mushroom field biomes tend to be heavily related to the ocean, this is very clearly not the case. The terrain-depended sub-biomes for oceans were kept in (like deep frozen ocean, deep warm ocean, etc.) due to their terrain generation still being biome-depended. However, the mushroom fields biome got it's sub-biome removed like every other land biome with the intention of the new terrain generation algorithm being able to create the unique land formations you would get out of one of these sub-biomes; it just seems that the mushroom fields biome got accidently left out during development, making it not effected by the terrain generator. In fact, not only are the mushroom islands still as flat as they were in 1.17, they actually got even more generic, as the removal of it's terrain-dependent sub-biome, along with this bug, makes the terrain feel even more static and blank, clearly indicating that something has gone wrong.