Zombies on Mooshroom Island

Please provide the seed of this world as well as the coordinates.

Seed: -1411370035

Coordinates of apparent spawning: 687 -627

Please don't put your description in environment.

Cannot reproduce.

Cannot reproduce either

I know this is old but this has been happening on my realm as well every since we started adding grass and building, especially while I was building around a Birch plank floor I laid, but they are not only spawning on the grass and planks; a creeper spawned on top of one of the mushrooms as well.