No Mushrooms are generating on Mushroom Island

I loaded a new survival world with the seed -6176126913539147506.

On the mooshroom island next to spawn weren't any mushrooms generating.

In Snapshot 20w10a and 1.15.2 the mooshrooms were missing,

but after I loaded the world in 1.12.2 the mooshrooms suddenly spawned.

The difficulty was set to Normal and Mob spawning was activated.