Highly relates to MC-107594, which is the same issue, but for corridors.

The bug

The top of some of the the high woodland mansion rooms have no air blocks, causing mountains, for example, to generate in the top of said rooms.

Seed: -1681023348413992357
Coordinates: /execute in minecraft:overworld run tp @s -31172.05 82.07 508.38 -534.10 11.36

Affected structures:

minecraft:woodland_mansion/1x1_as1
minecraft:woodland_mansion/1x1_as2
minecraft:woodland_mansion/1x1_as3
minecraft:woodland_mansion/1x1_as4
minecraft:woodland_mansion/1x2_s1
minecraft:woodland_mansion/1x2_s2
minecraft:woodland_mansion/1x2_se1

While all but 1x2_se1 are designed to also be placable on the lower floor, them generating on the higher floor causes this issue to arise.

1x2_se1 seems to have air blocks in the provided locations, some blocks that are not part of the structure still seem to get generated/kept on the top 4 layers of the structure.

The fix

For non-1x2_se1 structures, adding a 3 block taller variant should do the trick, not sure how 1x2_se1 happens though.