Relates to MC-203453.

Seed: -1681023348413992357
Coordinates: /execute in minecraft:overworld run tp @s -23625.63 105.93 -4571.51 -992.87 10.13

See screenshots.

While the rest of the wall layer is cobblestone blocks, a single cobblestone stair block is generated in its place when the room is at an inner corner of the mansion.

This only happens on the second floor, if the third floor has generated on top of it and there is an inner corner of the mansion walls.