Originally Posted by jaspergretsch
This may sound silly and obvious, but bear with me. When you unlock the phone, are you swiping up on the missed call notification itself, or on the little lock icon? The reason I ask is, if you swipe up on the notification (whether for missed call or text message or whatever), it OPENS that notification, which would mark it as "read", so it would then disappear from the notification bar.
If you swipe up just on the lock icon itself, it just unlocks to the main screen, and your notifications should stay in the bar.
This may not be what is happening, but it was something I thought of that could explain why you are seeing behavior that others aren't with this.