I noticed if the maps app is deleted along with the reminders app (I know they're not fully deleted- just not available on the home screen...) and SIRI is asked for map directions, she will make available the maps app in the app store- stating it is needed to fulfill the request.
Yet if a reminder is requested SIRI will simply state the reminder is set yet it truly is not.
For example, I asked for a one-minute reminder and after a minute passed checked spotlight and see I have no reminders even though SIRI stated it had been set.
Don't know if its a bug, yet surprised SIRI doesn't do the same as it does for the maps app and suggest the reminders app be re-downloaded while pulling up the app store (just like she does on a maps request).