Apparently, there is a recent change in Planner I was not aware of: when you create a new Plan, by default is configured not to send notification e-mails when a task is assigned / completed. To change this default setting:
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Simply Edit the Plan using the edit options available at the user interface.
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Just check the option that instruct Planner to send notification e-mails when a task is assined to a Plan Member or when a task is completed.