Yeah, that's happened to me before. For whatever reason, the DL app does not seem to cooperate when you're on standby. I've checked with airport agents more than once in the past to confirm I'm in fact on standby, and they've said yes even if it doesn't show in the app.

Where it falls apart is with mileage posting - keep an eye on that! I've had multiple flights not post when I clear standby before.