Having worked for a very large stock transfer record keeping agency for 30 years, it happens. It could very well be that the tape is just misplaced or mislabeled. It happens. Most backups end up in a cave (off-site location) for safety purposes in case disaster recovery is required. Some tape handling is done by data robots, but a lot of it is still done by hand. Mistakes happen.
I realize that it is unsettling to hear this news. I am also a Disney stockholder, but I'm not going to worry about it. Since BNY Mellon is going to pay for a monitoring service for shareholders, there is really nothing else they can do. It is very possible that the missing tapes will be found in the future. The same basic thing happened a number of times where I worked and it always worked out okay in the end.