What I expected to happen was...:
Zombies shouldn't spawn outside of the area of the spawner, or on a mushroom island if naturally spawned.

What actually happened was...:
Zombies spawn occasionally where they shouldn't.

Steps to Reproduce:
Using the world in the link provided https://drive.google.com/drive/folders/1saF0-Kd-t8q_IXEBJgkuDi2EgjiKHv-0?usp=sharing (because the attachment section doesn't seem to work properly with world files)
1. Stand in the coordinates 2610 64 -618
2. Wait as Zombies spawn in the spawner underground and go through the tube (The Zombies may need to be killed to reproduce the bug, incase the mob cap affects it).
3. At night time, Zombies will occasionally spawn in the general direction North-West of the player. (Note, this only occurs about once every two nights.)