This was an issue in the experimental snapshots and has carried over to 21w37a.

Sometimes mobs will spawn in biomes they aren't supposed to, because of what I assume to be issues with 3D biome placements. You'll see parrots in forests, wolves in plains, etc,

I'm not sure if every biome-exclusive mob has this issue, but it's definitely still relevant.

HOWEVER, flying either far above or far below the spot where these mobs spawned usually, in all of my tests, reveals the correct biome. So for example, far below the parrots in the birch forest is a jungle within the stone. This suggests that it's being caused by the new world generation instead of any previous issues.

To reproduce:

From gaspoweredpick;

Biome exclusive mobs also seem to be generating in the wrong biomes as well. From my observation in both the experimental snapshots and 21w37a, this issue only applies to animals that don't despawn.

To reproduce, use the seed -2350610002792544176 and run the following commands:

/execute in minecraft:overworld run tp @p -24 101 192 -164 12

Parrots in the middle of plains.

/tp @p -1513 67 2055 -157 21

Rabbits in the middle of a jungle edge.

What I expected to happen:

Biome specific mobs such as parrots and wolves to be limited to their appropriate biomes; jungles and forests/taigas, respectfully.