Why this happened, I do not know. Someone else will have to jump in to diagnose.
How to fix it? Try reading
and
to see if anything they recommend fixes the issue for you.
If this were my repo, if all else fails, I would run kopia snapshot fix invalid-files --verify-files-percent=100 --commit. That may or may not work. It worked for
Snapshot GC failure: error running snapshot gc: unable to find in-use content ID.