Stone, dirt, and leaves can appear inside Woodland Mansion

Is it just in the pumpkin rooms you find a stone block? Both screenshots show pumpkin rooms.

I guess so, I think it's only on pumpkin rooms

I've updated the summary to include dirt and leaves based on my own experience. Java Edition has the same issue (MC-228163) but I don't see it reported for Bedrock yet.

Could you give some examples of seed and coordinates where this occurs in 1.18?

I don't see any stones in pumpkin rooms too much now, but I see more leaves inside.

Leaves
Seed: -1412725903
Coord: 5723 89 1582

Sometimes on roaming in the other world, I can find dirt inside the mansion

& we can find grass blocks in prison cells 

Confirmed in 1.21.44

The stone and grass blocks generate because that's where the structure data mode blocks were that generate the mobs, they seem to replace them selves with the block that was there before the structure generated instead of air like they are supposed to.