When running a Minecraft server on Big Sur, Time Machine (Apple's built in backup software) often fails to properly backup world data. When attempting to restore from these bad backups, the world's chunk data ends up like scrambled eggs, with the console spamming 

[Server thread/ERROR]: Chunk file at , # is in the wrong location; relocating. (Expected , #, got , #)

for hundreds of chunks, utterly destroying the world with random chunk placement. 

Importantly, this issue does not occur on macOS High Sierra using the same server software and backup hardware. And yes I have tried multiple types of backup hardware and verified that Time Machine is working properly, the issue definitely revolves around Time Machine's interaction with the server specifically. I have also already reported the issue to Apple, though it is unlikely they will help much. 

Additionally, this issue seems to affect the 1.17 snapshots and modded server software (Spigot) much more often than vanilla 1.16.5, though I did confirm the issue for at least vanilla 1.16.5 and snapshots 21w19a and 21w20a.

Overall the reproduction rate is extremely sporadic and random, sometimes having 10 bad backups in a row, and other times having 10 valid backups in a row. The only reliable thing is that bad backups are always bad once made (and vice versa for good backups), so its not a restoring issue but instead lies in creating the backups. This also seems to be somewhat independent of whether the server is running or not when the backup is made, I've had it both ways, though if I had to guess I'd say the server being shutdown seems to have a higher chance of producing good backups (though not always). 

Apple did mention that "there might be something extra you have to do to make sure the data is not corrupt" and that the developer Mojang "would be able to let [me] know how to get it backed up." So there's that for what it's worth.