Notification for Completed Combined Notification?

Hello everyone,

We have been looking for a way to create a combined notification to be sent after all the tasks have been completed, in a similar fashion with the predefined notification of the assigned tasks:

So, even though the Combine Notification is Flagged

and the notifications of the tasks are like this:

Why doesn't the owner receive a combined email after all the tasks have been completed? Instead, he receives an email after each task is completed, creating a spam in the mailbox. Do you have any recommendations on how to send a combined notification email of completion?

Thank you,