Drive encryption with the device class parameter setting of DRIVEENCRYPTION=ON will only encrypt storage pool volumes. Thus, backupsets, DB backups, and export volumes will not be encrypted.
The mount continues, but encryption is not used.
If encryption is needed for this volume or other backupsets, DB backups, or export volumes, use a different method of encryption: use a device class parameter of DRIVEENCRYPTION=ALLOW and a hardware method of 'Library' or 'System' (do not use the 'Application' method).