I'm pretty sure there was never a formal engagement between Lady Isis and the Crown Prince. It was always just presumed to be that way because Isis' family spread that around and everyone acknowledged/accepted/wouldn't dare disagree with it because of how powerful the Duke's family was and how powerless the imperial family/Asher was. Lady Isis was marking her territory, claiming her self-determined "rights" so to speak. However, in this timeline, Prince Asher is able to become more powerful relative to the aristocrat faction with Aria's help, so that's why Prince Asher is able to confidently taunt Isis now.