Could it be that the dropping off the upgrade standby list arises because either:

1) it is a multi-segment itinerary with a domestic connection: the waitlisted GPU is removed at T-24 of the *first*, domestic, segment, but the airport upgrade list for the international segment has not been built yet (as it is outside T-24) when OLCI for the first segment is done. No checks are then performed to gather "lost" GPUs when the airport list finally opens.

or

2) OLCI (even within T-24 of the international flight) is the culprit and in such instances airport checking would be advisable... This could be the case if you did not get a boarding pass but rather a doc check message.

I always get doc-check with either OLCI or the chickens at the airport, so now I do agent check-in only. In the single instance I experienced the "expired waitlist", the GPU did correctly put me on the airport list after an agent checked me in... but this is just one case...