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Just confirmed that DesignSpecification:AirTerminal:Sizing is not useful in my case. I added it in the updated idf file (V23-2-0). The files are here.

DesignSpecification:AirTerminal:Sizing,
    DSATS,                   !- Name
    1,                       !- Fraction of Design Cooling Load {dimensionless}
    1,                       !- Cooling Design Supply Air Temperature Difference Ratio {dimensionless}
    1,                       !- Fraction of Design Heating Load {dimensionless}
    1,                       !- Heating Design Supply Air Temperature Difference Ratio {dimensionless}
    8.3142857143;            !- Fraction of Minimum Outdoor Air Flow {dimensionless}

ZoneHVAC:AirDistributionUnit,
    SPACE4-1 ATU,            !- Name
    SPACE4-1 Supply Inlet,   !- Air Distribution Unit Outlet Node Name
    AirTerminal:SingleDuct:VAV:Reheat,  !- Air Terminal Object Type
    SPACE4-1 VAV Reheat,     !- Air Terminal Name
    ,                        !- Nominal Upstream Leakage Fraction
    ,                        !- Constant Downstream Leakage Fraction
    DSATS;                   !- Design Specification Air Terminal Sizing Object Name

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The design air flow rate and the cooling capacity of the AirLoopHVAC VAV Sys 4-1 increased as expected. However, the outdoor air flow rate in the AirLoop is slightly higher than the minimum outdoor air flow rate of 0.42m3/s in winter and much larger in summer. It should be constant at 0.42m3/s. The chart below shows the outdoor air flow rate of the two idf files during SummerDesignDay and WinterDesignDay as an example.

image description

EnergyPlus is not good at autosizing multiple HVAC systems in one zone. Hopefully the bug(s) will be fixed as soon as possible.