Right now it waits a certain length before it deconstructs belts anyways, but only a few minutes. The idea was to save UPS by not have a huge list of belts to check if they are empty. Also so things don't disappear randomly hours later with no explanation. Maybe that isn't useful behavior and it should wait forever, and I can optimize the checking.
Was it a long belt and filled your outpost storage chests? I always stick a random requester chest in my outposts that empties onto the output belt. Mostly to deal with trash from the player when moving belts around, but also to deal with a case like this.