I understand that the segment leaderboards are having issues at the moment - but even before the current issues I've noticed this behavior. On segments I *know* I've crossed (since I created the segment from an activity), I will be dropped off the leaderboard. It seems to be the case only on segments that I haven't crossed in some time.
I don't mind if this is the expected behavior, I'm just wondering since I can't find it documented anywhere...