FWIW, the DisneyDiningBuddy notifications don't come right when an ADR becomes available, because they're not constantly pinging the system, apparently. Since I've switched my H&V ADR so many times and still have a notification set up, I actually get notified when my dropped ADR becomes available. The notification time has varied between 5 and 20 minutes after dropping before I get a notification.
Also, when I was coordinating a drop from another DISer, the ADR he was dropping, showed up, then disappeared, then showed back up about 20 minutes later. So I do think there's a delay in the system sometimes.