Was this a MR on a GUC? Would be a huge win to have SIN - LAX - AUS and then LAX - AUS is departing 5 min late, so use the rebook option in the app to do LAX - JFK (in Delta One) - AUS (F/J) all from one GUC... Maybe the agent thought that was abusive since outside normal fare rules. Doesn't make it right though..