It's been a while since I have used reminders, but I tried them out again today and noticed that I got a notification from the Alexa app about the reminder when it went off. I don't think it used to do this, was this introduced in the recent update? It definitely makes it harder to miss the reminder.
It does appear that, if you have multiple accounts on your Echo, you need to be logged in to the Alexa app with the same account you were using when you set the reminder to receive the notification.