The Successful Backup column contains the last date and time of the successful backups.
Backups with statuses Completed or Completed with errors are considered as successful backups.
Accordingly, Canceled or Failed backups are considered as unsuccessful backup.
The monitoring system tracks the last backup date starting from December 8, 2017.
Consequently, the Successful Backup Date value will be blank if:
The backup was not performed yet.
The successful backup event was manually deleted.
The last backup was completed prior to December 8, 2017.