Description: i have found a mushroom house and a miners house in the default plains super flat world even though i did not change anything in the biome option in the config for those two structures

Version:

Minecraft v1.16.5
Forge v36.2.30
Structure Gel API v1.7.8
Dungeons Enhanced v1.6

Screenshots:

I can't reproduce it
do you play with other mods?

yes
mod list:
AIReducer-1.16-0.1.0
citadel-1.8.1-1.16.5
dungeons_enhanced-1.16.5-1.6
EntityCulling-1.16.5-2.1.6
ferritecore-2.1.0-forge
iceandfire-2.1.9-1.16.5
jei-1.16.5-7.7.1.150
lycanitesmobs-1.16.5-2.3.3.3
Neat+1.7-27
RoadRunner-mc1.16.5-1.4.0
ScalingHealth-1.16.5-4.0.5+5
silent-lib-1.16.3-4.9.6
structure_gel-1.16.5-1.7.8
Waystones_1.16.5-7.6.4
XaerosWorldMap_1.20.3.1_Forge_1.16.5

and heres the config if you want
https://drive.google.com/drive/folders/1K4bM_5XwCnwer1e1UzKpE2eN66SJ2gFH?usp=sharing

this is a Vanilla bug, fixed in 1.18.2

I can't reproduce it,
I might try it again in the future but I leave it for now as it doesn't seem to be game breaking