That's because you probably didn't tell her you wanted to leave with her and are dating Mel.
You don't need to date Melody. Right now the potential endings are staying with Melody, leave with Amy, leave with Becca, leave with Isabella and maybe one with Sophia. Xianne doesn't want a relationship. So if you want a proper ending with Rebecca, best you can do is to tell her you'll leave with her and not be in a relationship with Mel.
From the walkthrough: "Note: If you’re on the Melody path, or if you chose to be with Amy, Becca will automatically break up with you."