Notification rules are global and apply to all arriving notifications of any kind.
If you are currently actively pinging the website in your web browser, you won’t get an email for a notification that arrives within a 10 minute window of the last time your web browser pinged the website.
To “guarantee” an email notification, either
close your web browser (or close the tab open to the website)
enable “mail me even if active on the website” in your user preferences