The bug
Reminder rings even after task is marked as complete.
To Reproduce
Steps to reproduce the behaviour:
Expected behaviour
No notification after the task is marked complete, even if the reminder is set to a latter date.
Screenshots

Info
Suggestion
I'm not a vala programmer but I guess that the reminder is tightly bound to the calendar.
Maybe by deleting moving the reminder entry from the calendar to a task-attribute-frame (a simple optimized location to store the properties of the task) can solve the issue.
I can reproduce it.
Even tried restarting Planner before the reminder notification appears. In case there's anything left in the cache that triggers the reminder. Nevertheless.
In connection with the sticky notification, this issue seems to be exciting as well.
https://github.com/elementary/gala/issues/725
That's a good point. Thank you very much for reporting it.
To solve this, it is only necessary to validate that the task is not completed. If it is, we do not send the notification.
May I add one more thing...
The Show Completed switch does not toggle back automatically. (I'm not sure if this can be reproduced...) Gets stuck... sometimes... (see the screenshot)

Edit: Yes reproducible, after we check ✅ and un-check a task.
That's a good point. Thank you very much for reporting it.
My pleasure.
To solve this, it is only necessary to validate that the task is not completed. If it is, we do not send the notification.
Okay.