[Christian]

Try setting all your flagged tasks to due today, 5 minutes from now (10 if you have many flagged tasks). Then close the app and also close it from running in the background (double click on home button, tap and hold OF icon till it move, press delete icon).

From what I have observed, OF reminders actually act as reminders right now, not as a kind of an additional perspective in a different place. Hence it shows the reminder but when you then open the corresponding task, oF assumes that you are "reminded" now and deletes the reminder from notifications center. That could be another thing to add to your support email: please leave reminders for due, due soon and flagged items in notifications until they have been marked as done in the app.