The Bug
Grass and sugarcane were recently added to the list of items that are able to be placed on Mycelium, but the Mushroom Fields biome will now naturally generate with the occasional grass and sugarcane patch at it's shoreline.

Reproduce
Create a normal creative world in 21w43a with the seed: 5666451149799436100
Fly to 8528 / 65 / -1753 and face east
Look for sugarcane on the mycelium shore

Expected Result
Sugarcane and grass should not generate in the Mushroom Fields biome