Do you happen to know of any events where a page that was archived was removed afterwards? Just curious, haven't experienced this issue myself but have heard it happened a couple times in the past.
There are 2 cases:
1. some pages are removed by authority requests. Metadata and the name of authority are still there (example https://archive.is/6t3I2)
2. sometimes people share links when the archiving process is not yet complete. if the process fails (for numerous reasons: from browser crash because of too long page to permanent redirect to /login.php), then there will be “Not Found“ page without any metadata or explication. I should keep and display info about such failures, but it is not done yet and it might be perceived as mystical disappearance.