Chunks regenerating even though they have been explored.

sometimes when I reload a world, some chunks I have explored change even though I didn't do anything to them. It's best visible when you use a map. When you fly over a previously mapped area the map changes. It has removed two places in my world where I planned to build something. For some reason however it only affects chunks I have built in yet. But from some of my builds, the new chunks can be seen and it changes the context of everything.

The expected result is that reloading the world doesn't affect already generated chunks. Nothing should change.

The observed result is already explored chunks changing without me wanting them to.

Last time it happened after updating the game to 1.21.100, but it also occasionally happens without an update to the game so it seems like a general issue, not update related.