chunks move downwards by 4-5 blocks

In a world (~80 MB), I've recently noticed large sections of the world have randomly moved down by 1–5 blocks. This doesn't seem to affect underground, only the top layers of blocks. Areas I have built in haven't been affected, but I often notice when updating a map etc that sections around the edge are refreshed.

Video of this: https://drive.google.com/file/d/1IJbVldlv907w-jpLy-Glk3unOLWaEu79/view?usp=sharing