This has been an issue for a very longtime, in any world spawned where a Mushroom Biome is generated there will often times be only a single Mooshroom for the entire biome or none at all. Sometimes you may find a Mooshroom and then have it despawn on you at a later date, never appearing again.
*Locate a Mushroom Biome
*Find a Mooshroom
*If Mooshroom is present, it will be the only one present regardless of the size of the biome
*Upon finding the Mooshroom, leave and unload the chunks
*Wait for a few in-game days, return
*Mooshroom will despawn
Alternative Steps 2:
*Find Mushroom Biome
*Find no Mooshroom, Mooshroom never spawns regardless of time waited
I know there is a spawn limit for mobs, but why is the limit so small for Mooshrooms? Shouldn't at least two spawn so that a player has a chance to breed them if they like?
Test Seed: -284047900
Coords for one Mushroom Biome close to spawn; no matter how many times I have used this seed, this one has never seen even a single Mooshroom:
Coords for the other Mushroom Biome; where a single Mooshroom spawns or doesn't spawn at random every time I use this seed:
Further seeds in comments.