The Mushroom Fields biome does not generate pumpkin patches. The pumpkin patch feature can only generate on grass blocks, which are intended to be absent from Mushroom Fields.  However, the vanilla biome JSON file for Mushroom Fields specifies the "minecraft:patch_pumpkin" feature.

The presence of this pumpkin patch data in the Mushroom Fields biome can be confirmed by navigating to data/minecraft/worldgen/biome/mushroom_fields.json

The requirement for grass blocks for pumpkin patch generation can be confirmed by navigating to data/minecraft/worldgen/configured_feature/patch_pumpkin.json