This issue has been around during the entire Snapshot Phase for 1.17

I believe this error ties closely with the Furnace/Chest/Chunk Errors other are having. (As we have had all those issues as well across the snapshots)

Currently when players join our server some chunks will fail to save (Let say you made a change in the overworld, then either logged out or went to the nether, in some cases the chunk you leave will fail to save)

A temporary solution to counter this problem is to force /save-all in a datapack before teleporting (But getting this to trigger on logging out is near impossible to counter) 

Has anyone else experiences Chunk Saving issues in snapshots?