That error usually indicates a problem on the host creating the backup. (Or in some cases on the host running the checks. Does restic check --read-data-subset 95/256 continue to return that error?).
This error can only occur when the affected blob was damaged during creation of the pack file (or while checking it). This is usually caused by bitflips in memory, so please run a memtest on the affected host. Which kernel version are you using? There have been problems with older version of Linux 5.2-5.4, see also Troubleshooting unreproducible consistency errors - #19 by cyounkins.
If the error disappeared then its usually some problem with the host verifying the backup, assuming that the damaged blob was not removed in the meantime by running prune.
That depends very much on the damaged blob. It might as well be a blob only used for a single file in a single snapshot or a blob used by hundreds of file in every snapshot. You could use restic find --pack 5ef6ac9a to get an overapproximation of possibly affected files. This will output too many files as it will report every file which uses any of the blobs in the pack file and not just for the damaged blob.
My guess would be that it was either a bitflip while checking the backup or that the pack file has vanished in the meantime.