Once Did you run a model with HVACTemplate objects, it is a good idea to verify that the inputs field System Availability Schedule Name, Supply Fan Maximum Flow Rate and Supply Fan Minimum Flow Rate for all objects in the expidf to make sure that the inputs in the HVACTemplate objects get translated as desired.

I would suggest that you make a copy of the expidf and verify all inputs related to flowrates of the following objects:

group DesignSpecification:OutdoorAirHVACTemplate:System:VAV - just to make sure that the OA flowrates are modelled accurately;

AirTerminal:SingleDuct:VAV:Reheat - I correctly? You would used the option FixedFlowRate for the field of Zone Minimum Air Flow Input Method and add need to hardsize the max and min flow rate flowrates in these objects and apply the respective fields;appropriate schedule in this object.

As per the I/O ref guide, Sizing:System is used for calculating system sizing for every design day in the IDF and the max heating & cooling air flow rates and capacities are saved for use in the component sizing calculations.

Did you verify that the field System Availability Schedule Name, Supply Fan Maximum Flow Rate and Supply Fan Minimum Flow Rate for all objects in the group HVACTemplate:System:VAV are modelled correctly? You would need to hardsize the max and min flowrates in these objects and apply the appropriate schedule in this object.

As per the I/O ref guide, Sizing:System is used for calculating system sizing for every design day in the IDF and the max heating & cooling air flow rates and capacities are saved for use in the component sizing calculations.