Growing huge fungi replaces all solid blocks in growth path.
To repeat:
Place respective nylium
Plant respective fungus
8 blocks above nylium block, place 3x3 grid of any solid block (I used cobblestone for images)
Use bone meal to grow huge fungi.
Remove nether wart blocks above 3x3 grid
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We're actually already tracking this issue at MCPE-65661.
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