Top of high woodland mansion corridors do not replace old blocks

No it's not; this is a certain section of blocks not replaced with air, that is post-structure generation blocks being placed inside.

It's actually the same issue. Grass generates, and then structures don't replace blocks inside them with air.

As shown in MC-223305, structures generate after plants. Otherwise, you wouldn't see only the top part of tall grass generating.

I suggest keeping either this issue or MC-140727, and using a better name, such as "Some structures don't replace terrain inside them with air".

The fix is correct that a woodland_mansion:corridor_floor_hight.nbt piece needs to be made that is 3 blocks taller. In code, within the WoodlandMansionPieces$MansionPiecePlacer.createMansion method,

list.add(new WoodlandMansionPieces.WoodlandMansionPiece(this.structureManager, "corridor_floor", blockPos3, rotation));

should be replaced with 

list.add(new WoodlandMansionPieces.WoodlandMansionPiece(this.structureManager, (floorIndex == 0 ? "corridor_floor" : "corridor_floor_high"), blockPos3, rotation));

That way the second and third floors of the mansion can use the taller corridor floor piece that can replace the terrain in the 3 block taller hallways. 

 This problem exists in 1.18 Pre 7. In one area the terrain generates floor to sealing even taking out part of the floor. Also added pics of a bunch of areas I found. Seed is 4132146605620838058 on pre release 7

Also terrain is generating on the lower roof on the outside.

That screenshot is a different bug than this one (MC-107637); there's no structure piece at all being placed there.
It would be best if you added that screenshot there, and removed it here.