With some of the most recent snapshots, a new spawning algorithm was added to avoid players from spawning in the ocean, however, it seems like it's no longer possible to spawn in the mushroom fields biome.

In most seeds, the player spawns near the center of the map (x=0 z=0), but that can change if the center of the map is in an ocean.

SEED: -1612657249 / Zoglin 

In this seed, the player spawns around x=-530 z=20, which is in a shore right next to an ocean, and by teleporting to x=0 z=0, there is a very large mushroom fields biome right at the center of the map.

/execute in minecraft:overworld run tp @s 0 ~ 0

Spawn point:

Mushroom fields at the center of the world