Hi Rodolfo,
Can you attach a screenshot of the email you receive after reboot and a screenshot showing the alert details for it (to check whether it is really closed, and whether the creation date is old and not from the recent hours). Also it looks like you have been reproducing the issue, so can you send us the log bundle.
Some internals: the notification are queued and persisted on disk, so in very extreme case when sending mails is extremely slow or there is some issue sending out mails (blocked after some mails, stuck, etc) – and some amount of queued mails are sent out after restart (old mails) – but this is only an idea.
Thanks