Structures generate floating in air after world upgrade

The bug

After upgrading a world from 1.17.1 to 21w43a, some structures generate floating in air.

Steps to reproduce

1. Create a world in 1.17.1 with the seed 1304268445 and render distance set to 12 OR download the attached 1.17.1 world 2021-10-27_14-06-20_Floating structures.zip

2. Upgrade the world to 21w43a

3. Teleport to the following locations and observe floating structures

/execute in minecraft:overworld run tp @s 202.70 110.11 23.82 19.72 13.91

/execute in minecraft:overworld run tp @s 78.47 112.05 179.88 44.53 17.44

Notes

Note that this is likely related to world blending; there is terrain beneath the structures when recreating the world with the same seed in 21w43a.