how is calculated the moment to display a notification ?
I imagine joplin uses todo_due but does it raise the popup just once at the right moment ?
What happened when the date is due and the notification has not been shown at the expected date &hour ?

It uses a timer to launch the notificatoon at the right time.

If it didn’t work it’s either a bug in Joplin or in the OS notification system.

Did you watch a cartoon during this reply? :))

Otherwise there is no bug. I am looking for the right way to do it for joplin-web (to popup a notification) and jong-toolkit (to send a mail)