Fungus trees don't replace Nylium with Netherrack when growing

Grass blocks decay to dirt after a while when a solid block is placed upon the grass block. When a sapling grows to a tree on a grass block, this happens immediately. Nylium seems to decay to netherrack when solid blocks are upon so it can't be more that logic that fungus trees will immediately replace nylium below with netherrack, but that doesn't seem to happen.

To Reproduce

1. Place a nylium and a grass block with a small space between them (let the trees be able to grow)

2. Place a small fungus on the nylium block (don't have to match) and a sapling on the grass block.

3. Grow the sapling with bonemeal
   ✔  Grass block turns into dirt immediately after growing the tree

4. Grow the fungus with bonemeal
   ❌  Nylium remains after growing the fungus

5. Do /gamerule randomTickSpeed 1000 (except if you're patient enough to wait)
   (!) See the nylium decay below the fungus tree after a while

The same issue happens during world generation:

1. Do /gamerule randomTickSpeed 0 to prevent nylium from decaying

2. Find a warped or crimson forest

3. Destroy the lowest stem blocks of fungus trees and see the nylium below the stem

(blame the enderman, he stole my dirt block 😛)

Performance

The nylium decays after world generation which causes many chunk sections to be re-rendered unecessarily. On old and low-performant computers this causes quite a lot of lag. Nylium should either decay immediately below fungus trees during world generation or should not decay at all (in the last case other bugs apply 😉).

Even more weird is that this issue is being reported multiple times and they are already resolved...? The issue still applies in 20w13b...

• MC-173321

• MC-170835

• MC-171475