Had 2 different tasks each with a reminder for the same time time today. Only 1 reminder occurred, the other task still shows reminder icon but it never happened.
Are you 100% sure they were created the same, as in both were exactly "today at 2pm" for example?
And you didn't see it in the notification area either, maybe it didn't make any noise due to the other one that came in?
I will have to watch it next time. Pretty sure they were the same. Definitely not in the notification area.