For a ducted system to cool an area effectively (and hold temperature overnight), the air conditioner must be correctly sized and the ductwork/zoning must be designed to match the heat load of the space. If the capacity is too small for the area being conditioned, you may still feel airflow at the vent, but the room temperature won’t reduce or maintain setpoint—particularly on warmer nights.
A common setup is a day/night zoning arrangement, where the day zone (living areas) and night zone (bedrooms) are intended to operate separately. In many cases, the system is only sized to comfortably run one zone at a time. If your system has been designed this way, it would explain why the downstairs open-plan area cools well when operating, but the upstairs bedroom zone may struggle if the available capacity/airflow is not sufficient for that zone.
Another factor can be the location of the return air/sensor (or controller sensor). The installer typically determines where this is installed, and if the sensor is located in an area that cools faster (or is influenced by different airflow), the system may think it has reached temperature and reduce output, even though the bedroom remains warm.
To confirm what’s happening on your installation, could you please provide:
A floor plan (or a simple sketch is fine) showing the zones, outlets, and return air grille locations
The make/model of the ducted unit (and if possible the outdoor unit model as well)
Once we have that information, we can review whether the unit capacity, zoning design, airflow allocation, and sensor/return location are appropriate for cooling the upstairs bedrooms overnight.