The minecraft:woodland_mansion/carpet_south_2.nbt piece has an air block set where there should be structure void. The result of this is many woodland mansions where the 2nd floor meets the 3rd floor for the outer walls may have holes as seen here:
[media]
Here's where the air block is (on the right side with the column of Structure Voids) in minecraft:woodland_mansion/carpet_south_2.nbt that needs to be turned to Structure Void. Doing so with a datapack resolved the issue and the holes could no longer be found in mansions.
[media]Example seed of the issue:
Seed: 100
Location: -27410 78 98665
Datapack that replaces the carpet south 2 nbt piece to fix the holes to show that the piece was the issue:
Linked issues
Attachments
Comments

@Avoma Err. I would split MC-110098 into two separate bug reports. The 3 block high hole is caused by piece placement ( from what I can tell) which is an entirely different problem. This single block hole is caused by an air block in an nbt piece.
Please do not mark this as duplicate and instead, make that bug report focus on the 3 block high hole while this one focuses on the single block hole.

Affects 24w21b.
Seed: -495754591562773570
Coordinates: /execute in minecraft:overworld run teleport @s -962.94 108.73 -2271.04 85.65 12.63
[media]
Found another, larger hole in a mansion.
Seed: 464728820865257591
Coordinates: /execute in minecraft:overworld run teleport @s 432290 137 -6041 -1002 29
[media]
I also found another instance of this issue happening in my Survival world a month ago and made an issue on it, which was resolved as Won't Fix by Mojang. I didn't see this bug report.
The bug report in question is MC-271339.

Confirmed in 1.21.3

Affects 1.21.5:
Seed: -3876154560102903575
Coordinates: /tp @s 448 84 -2065
Duplicate of MC-110098