The bug

Using mouse 3 / pick block in Creative on mushroom blocks gives the corresponding mushroom plant item instead of the mushroom block item.
Same with Survival: If you have the block in your inventory using for example silk touch, pick block won't pick it.

Since a similar bug with farmland (MC-95469) was fixed this should be a valid report.

How to reproduce

1. Give yourself some mushroom block items

   /give @p red_mushroom_block 2
   

2. Place one
3. Middle click / pick block on it
   → In Creative you get a red mushroom plant; in Survival it does not move the block to your hotbar