If I'm not mistaken only mooshrooms are supposed to be able to, but im getting quite a few of everything that normally spawns in the overworld on mycelium, is this intended?
If i may offer a possible explanation...Mycelium, generally when in its natural biome, spawns only mooshrooms and no other mobs, thus the mushroom biome is deemed peaceful. However, in the case that a player takes some mycelium from its natural biome and transplants it somewhere out of its natural biome, then both hostile mobs and passive mobs can spawn on it, as if it were grass. So basically it is the mushroom biome itself that prevents mob spawning. The mycelium, alone, can produce mobs if placed outside of the biome.
Only other possible reason i could think of is if the server has some kind of glitch, bug or artificial mob spawning (through a command or plugin). However, I think the first explanation is more probable.
Edit- If you are actually in the mushroom biome and this event is happening, then it must be explanation 2.
