Mobs that should spawn on specific biome sometimes spawn on the biome connected to it

Group of mobs sometimes naturally spawn on the biome connected to it. I cannot say how to reproduce it because it happens naturally.

What I expected to happen:
Mobs should only spawn on the biome.

What happened:
Mobs spawn on the biome connected to the biome mobs should spawn.

The seed: 349518308
Coordinates: X: 519, Y: 69, Z: -323