Ever turned in your iPhone to see a calendar notices suddenly disappear from the lock screen? I've been seeing this a lot lately, and it's really bugged me – especially when my phone is on silent (as it is most of the day), so I can check it at chosen intervals rather than be disturbed (developer thing, no doubt) .
Well it turns out iOS will remove a notification after the event has finished. Kind of makes sense, although it does mean if you totally miss an appointment, you'd be none the wiser unless you pay very careful attention and see it before the notification quickly disappears.
Odd quirk, but probably not a bug – just a design choice (I would say it makes sense to remove them from the notification centre, perhaps not the lock screen) and Apple should probably hide it before the screen turns in to avoid the confusion of seeing something for a split second.