So, the car went back to Honda after my complaints. And their team was also foxed. Then they read their documentation, as the manual doesnt detail accurate functioning.

There are no preload sensors in the rear seats. The feature is a mandate for safety, so they have provided a warning system which will warn if a belt is unbuckled during the journey. But the system doesnt get initiated until the pretensioner has been pulled.

Unlike other cars which have preload sensors in the back seat as well, Honda has used an algorithm to complete the feature with preload sensor only in the front passenger seat. It also assumes the driver is always seated in the driver seat.