How to reproduce
Locate a mushroom room, either by finding one naturally generated:
/locate structure minecraft:mansion
Or by spawning one in directly:
/setblock ~ ~ ~ structure_block{mode:"LOAD", name:"woodland_mansion/1x2_a7"}
→ ❌ Note that no mushrooms spawn.
Linked issues
is duplicated by 1
relates to 2
Attachments
Comments 28
Can confirm in 20w51a.
Can confirm in 21w06a using the provided seed & coordinates.

Is probably caused by MC-213695

Can confirm in 1.17.1.

Loaded up a world in 1.20, and somehow the mushrooms are present.
Seed: 611
Location: 274 67 412

I saw an another room that mushrooms are present in 1.20.1 besides already mentioned case. And I tried to generate it with structure block, and it had mushrooms. It dropped as item immediately tho - this might be brightness issue I think.
It is, order may have changed slightly in 1.20 so that the mushrooms are in darkness before their brightness check, rather than checking immedietly when placing the structure block by block.
Also tested in 1.20.1 and two mansions I went to had the room and generated with them.
EDIT: It may have been a mod that was fixing it....

Confirmed in 1.21.3
Can confirm in 20w49a. Absolutely no mushrooms are generated here.