>production box only contains data from the last 3 days, but the backup contains data from the last 12 months
Even if the source can only perform new backups, it's a timing attack with a deduplicating system. The attacker can attempt to back up chosen data to infer properties of the existing backups.
You can remove this only by removing deduplication (or by crippling deduplication to work only at the server-side, and incur wasteful network requests)
Even stricter access controls (write once, no read) might help with that. Not sure if you can do that with S3 though.