Many biomes like forest, swamp and plains try to generate mushrooms and contain the features "minecraft:brown_mushroom_normal" and "minecraft:red_mushroom_normal" for this, however this does not happen as mushrooms cannot be placed on the grass block or dirt.
Comments 4
The mushroom_grow_block tag only applies to blocks where you don't need darkness to grow a mushroom with bone meal. It has nothing to do with world gen.
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