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.)
Comments 4
@ampolive
It makes sense that it would be that feature, though the zombies only ever seem to spawn in the one direction, while any spawning in other directions have spawned off the mushroom island and pathfind towards me. Regardless, it seems unusual for this to occur on a mushroom island where mobs don't naturally spawn.
I just tested and zombie reinforcements can spawn on mushroom islands. It could be an oversight, so I created a new ticket: MC-227735.
What you are experiencing are probably zombie reinforcements.