This is a minor issue and does not have impact on actual gameplay.
The stairway of a woodland mansion going from floor 2 to floor 3 sometimes generates one block too far away from the ground of the 3rd floor; or the ground generates one block too far. This is a little bit ugly to look at.
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Duplicate of MC-107637
MC-107637 didn't show up in search though
I feel like I've seen this for a really long time, but I can't really recall when. I've questioned it before, but never thought of reporting it. It is a weird and slightly ugly issue.