My only thought is (pure speculation here), with the earlier flight being wide open, the system automatically cleared you from the standby list because UG inventory opened up in the GLD window.
It goes against the published policy, but I wouldn't be surprised if you benefited from a system error which rarely manifests itself due to the rarity of UG inventory being available for non-EXP pax inside T-24, but before T-4.
I usually have the opposite problem with flight changes, whether standby or IRROPS reaccom, where my upgrade requests are not carried over to the new reservation. Perhaps the automated system does a better job of retaining those.