Why Google Drive backups created by Plesk administrator are not shown in Subscriptions > example.com > Backup Manager?
By design, the backups stored on Google Drive as an administrator should not be shared with Customers in their subscription.
If you believe that domain owners needs to have access to backups created by Plesk server administrator vote for this feature on Plesk UserVoice.
The top-ranked suggestions are likely to be included in the next versions of Plesk.
As a workaround, perform server level backups storing backups in local and remote storage as described here How to configure Plesk to store backups both in local+remote storage