fsck_apfs fails with “btn: invalid value” and “Snapshot is invalid.”

Question

I have an old MacBook Air running out of disk space, and DaisyDisk reports that 25 GB are taken by "still hidden". Following their suggestions, I've tried to run Disk Utility First Aid as well as a raw sudo fsck_apfs -ln /dev/rdisk1s1, and both give me the following error:

...
** Checking the object map.
** Checking the snapshot metadata tree.
** Checking the snapshot metadata.
** Checking snapshot 1 of 1.
error: btn: invalid value (3808, 20)
   Snapshot is invalid.
** The volume /dev/rdisk1s1 could not be verified completely.

I've checked both sudo tmutil listlocalsnapshots / and sudo tmutil listlocalsnapshotdates /, and both show no results. I've tried booting into recovery/single-user mode and running sudo fsck_apfs -y /dev/rdisk1s1, but it fails with the same "snapshot is invalid" error.

Is there any way to get fsck_apfs to successfully run and repair the disk? If not, as a last resort, would it make sense to fully delete this machine's Time Machine backup and recreate it from scratch as a way to try to get rid of this invalid snapshot?

Edit: I also forgot to mention, I have disabled Time Machine backups and restarted as well (in an attempt to get rid of all local snapshots).

Answer 1

See the comments above for the approach I wound up taking. It appears Apple's tools don't allow recovery of the filesystem in this case, so I had to back everything up and reformat/reinstall/restore. That seems to have fixed the filesystem errors and disk space problems.