Arelith

When attempting to plant a mushroom seed in any suitable soil, including soil that has room or has reached its capacity, the server log will say "The soil is not suitable for planting this type of seed" and nothing happens.

Other seeds work just fine.

Some locations do not accept all seed types. Surface can't always support UD seeds and vice versa. Can you confirm the exact area(s) attempted, and exactly what seed you were using?