This appears to happen when there is an automatic cleaning process and the specified cleaning tape has expired. The next tape that enters the drive gets marked as a cleaning tape.
You will see the following entries in your logs:
:: TapeDrive alert: The tape drive is due for routine cleaning:. Wait for the current operation to finish. Then use a cleaning cartridge. Check the tape drive user manual for device specific cleaning instructions.
:: TapeDrive alert: The last cleaning cartridge used in the tape drive has worn out:. Discard the worn out cleaning cartridge. Wait for the current operation to finish. Then use a new cleaning cartridge.
:: Marking tape <TapeName> as a cleaning cartridge You will notice a slot in your tape library has disappeared.