Large nether fungi, upon world generation/when grown by bone meal, do not immediately convert nylium below to netherrack

The bug

Fungus trees grown (naturally or using bone meal) from crimson or warped fungus do not immediately convert the nylium below to netherrack.
Instead it converts on its own after some random time when it notices that a block is above it.

This is inconsistent with regular trees which immediately convert grass to dirt.

How to reproduce

1. Plant a fungus on the respective nylium
2. Use bone meal on the fungus to grow a tree
   The nylium below is not converted to netherrack immediately