Also have this issue with 1.20.30, i tryed reverting to an older copy of the world but got the same issue in the same place.
played with the world in creative, when i removed entities and added entities, pillagers, horses and was fine until i added a villager, then the area started to drop down.
Game has trouble processing things around the village chunks ( like save game lag)
Note: Old world and area is old chunks
Fixed!