Mansion generates with big hole in wall to roof transition

Can confirm that this is an issue. Also affects 20w49a.

Can confirm in 20w51a using the provided seed and coordinates.

Can confirm in 21w03a.

I can't seem to find any mansion that is affected anymore in 1.18.
I found mansions with this wall-roof transition at hallways, walls between rooms, and in the middle of larger rooms, none seem affected.

The only problem that seems to be left is a single missing cobblestone stairs block/the wall's pillar not continuing downwards.

Actually, this seems to be a duplicate of MC-110098 that nobody noticed.
I'll copy my comment to that report.