PER originating fare may not be the same as SYD... CX fare rules allow R+V for PER-HKG-CCU doesn't mean R+V is valid for SYD-HKG-CCU... Maybe E+V is OK, R+M is OK, but not R+V or E+M... Departure date (Weekday v. Weekend) also matters

Anyway, just an illustration, I don't know the real reason behind

If that's really fare rules issue, even DMP has to follow... Why not ask reservation directly what's the lowest possible combination? If R+V is a no-go for your itin, how about R+L? R+M? etc etc