*** UPDATE IF YOU HAVE THE SAME PROBLEM ***
It turned our the problem was Windows "Quiet hours" which was muting the notification the moment you tried to display it, meaning it never actually gets shown to the user. Turn off "Quiet hours" and the notification displays normally.
Of course this doesn't help much if you're trying to develop an extension because you can't ask the user to turn off "Quiet hours", but at least you know what was causing the problem.