As the title says
The issue has occurred since 24w05a, and can occur even if the world has been played in a newer version in chunks that haven't been loaded since before 1.9.
Steps to reproduce:
1. Generate a world in 1.8.9 or earlier
2. Open the world in 24w05a or later
Observed result:
Many chunks get regenerated. Which chunks get regenerated remains consistent when loading a backup, but I can't see any pattern in which ones do.
Provided screenshots show a world in 1.8.9, with a layer of glass added for visualization, and the same world after loading in 24w06a, showing holes in the glass where chunks were regenerated.
Please provide consistent and clear steps to reproduce this issue. A helpful format you can follow can be found here. It would also be very helpful if you could provide some more context as to when/how the chunks are regenerated if you're able, because random or inconsistent behavior can be hard to confirm.