FYI notification for resignation Submission not triggering in mail

The FYI notification for the resignation( when submitted by employee) is not getting triggered .

The requirement is that when the employee submits the notification, it is first going to the HRBP,HR Head and few other AORs as FYI. Also, it goes to the manager for approval. Approval is triggered to manager without fail. However, FYI is not sent.

Task aggregation is set as once per task now.

I would also like to understand the difference between task aggregation in simple terms.