Ok, so there is a parent snapshot, which is good. I suspect that the device ID is changing, which could happen when the source filesystem (/media) is unmounted and then mounted again. To verify this, we would need to look at the tree objects for both snapshots. You can see in this snapshot what the tree ID is (ad2f1c...) as well as the ID of the parent snapshot (3f9994...).
Can you post the tree from this snapshot (restic cat blob ad2f1c9fc3b907e972c7920920c943c8f48fa845ca50325521f66f00b41f0a5e) as well as the tree from the parent (restic cat snapshot 3f999436d9465215f0199e6663f5bba9f2a505c6e1401c1923da68273d031c1e, look for the “tree” line, and restic cat blob $TREEID)?
If this contains sensitive information you can private message the output to me to keep it out of the public forums.