Blocks of giant mushrooms don't send block updates after growing

The bug

When a mushroom grows into a giant mushroom, the blocks the giant mushroom consists of don't send block updates to adjacent blocks. Observers trigger properly though.

This happens both with red and brown mushrooms.

To reproduce

1. Build the test setup seen in this screenshot:
   
2. Place a red or brown mushroom on top of the podzol
3. Use bonemeal on the mushroom until it grows
4. The Redstone lamp won't activate:
   

Code analysis

A code analysis by [Mod] NeunEinser can be found in this comment.

Credit

Discovered by docm77 in this video