Using classic Outlook for Windows in business environments
This fixed the problem for me, thanks.
I had a meeting which I had attended, but I didn't get around to dismissing the reminder.
Per April's post, I switched from the mail to the calendar, and closed outlook via the close X in the upper-right corner of Outlook's window. I didn't bother to do anything with the reminder window, I left it visible. When I clicked on the X ("the close box") the main window closed and the reminder closed, too.
Then I went ahead and opened Outlook again. The reminder wasn't visible. However, as you guys will remember, if you don't dismiss a reminder, it will come back, and so this one did, and it was displayed in a proper reminder window with all necessary pieces visible.