When mooshrooms try to respawn randomly they should check for mycel below them - because that is the thing they live on. But they currently check for grass, that does not even exist on mushroom islands (the only biome mooshrooms can spawn in)..
Reproducing: Kill all mooshrooms on a mushroom island - they will not respawn. Now plant a lot of grass there (replace the mycel by grass) - they will respawn.
This is the check in the decompiled sources, that should probably be overridden for mooshrooms:
https://github.com/Bukkit/mc-dev/blob/master/net/minecraft/server/EntityAnimal.java#L212
Linked issues
Comments 3
Also, Mooshrooms are not randomly respawning on Grass in the Mushroom Island Biome - Only once at creation of the biome.
I have used the search function, but did not find it. And I don't think that it really is a duplicate, because the bug I addressed is about the random spawning, not about spawner-spawning.
And they definitively are randomly spawning on grass in a mushroom island biome. I tried that. You will have to wait some time to see that, because the random spawning is very slow.
Duplicate of MC-7463
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