That is indeed very challenging.
From a Commvault perspective snapshot references will only be cleaned up from the CommServe DB, when the API call is returned by AWS.
In this case it sounds like a potential edge condition, where AWS has already sent the acknowledgement to Commvault and cleanup is completed and updated in the CSDB or someone has manually force deleted the Snapshot reference in Commvault (snapshot consolidation), without cleaning up from AWS.
Since the snapshots are over 6+ months old, i dont think we will have sufficient logging to identify the cause.
However would still recommend to open a support ticket to dig in further.