Wanted to add my frustrustration with this behaviour too. From reading back through, I understand that if a meeting is missed (i.e. current time > meeting start time), the reminder on that meeting is set to 'none', regardless of whether a reminder was automatically set according to default preferences, manually in the meeting item itself, and regardless of whether the original reminder is dismissed or not.
What makes this behaviour especially frustrating is that it specifically targets meetings you have missed! I rely on meeting reminders quite heavily. If for some reason I miss a meeting (away from my desk, otherwise engaged), I often - quite logically - need to apologise profusely to the other invitees and the meeting is rescheduled by whomever set it up (note it doesn't matter whether it's my invite or someone elses). Then, when future meeting comes along, no reminder so miss the meeting AGAIN.
Missing the meeting once is barely excusable, but missing it twice is embarrassing. And that embarrassment could easily be avoided if meeting reminders reverted to default when they're rearranged from the past into the future.
Exceptionally frustrating behaviour and desperately needs fixing. Unfortunately though, it will never be upvoted because it's low frequency - and therefore most who are affected probably don't realise why.
MINOR WORKAROUND: This is less than ideal (and really annoying you can't save an advanced find in Calendars in the same way you can save Search folders in Email), but you can at least find meetings that are affected. Shouldn't need to be done, but here we are...
Go to your calendars, select search -> Search tools -> Advanced find
In the window that pops up, click the Advanced tab and add these three fields:
Start -> On or after -> "today" (without quote marks, but you literally type in 'today' into the box
Reminder -> equals -> no reminder
Categories -> doesn't contain -> "Holiday" (this is a specific one in my business where shared calendar shows all national holidays and events - there's practically one every day so don't want reminders!