Very rarely zombies spawn in a mushroom biome.
I have an enclosed dark area, 5x5 large. There is no way in or out unless I break a block. It is in the center of the mushroom biome, and I've checked every space with F3 to check if any blocks were not considered mushroom_fields. A Zombie should not be able to spawn in there.
It does not happen often. This has happened only twice in my time playing with the new snapshots. So my guess would be that this happens maybe once every 50-70 hours of gameplay.
It is a survival world.
Comments 3
There are villagers about 150 blocks away, in an inaccessible area. I have a villager trading hall in a floating island I constructed. None of my villagers are missing.
I did use the F3 screen to make sure. Every block in that enclosed area is mushroom_field. The enclosed area is inside of my base, which is close to the center of the mushroom biome.
The two times the zombie spawned it was in different locations.
The first time it was inside of an enclosed house, with iron gate doors as the only point of accesses (much larger space, but I did check with F3, it is all mushroom_field).
The second time was in that much smaller 5x5 100% enclosed area inside my base. That confirmed my suspicion that it was a bug.
Is there a villager nearby? Also, are you sure the zombie didn't spawn in a different biome? Use the F3 screen to make sure.