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It sounds like the total supply air flow for the AirLoopHVAC is higher than the outdoor air flow, thus mixing of some of the outdoor air with some of the return air. Have you checked the "Air System Outdoor Air Minimum Flow Fraction" output variable?

I have found that the way outdoor air is controlled in AirLoopHVAC is not very intuitive. If you are using variable volume zone air terminals, then the amount of air required at the air terminals drives the system flow. For most DOAS systems, this is the easiest approach, as it is easy to schedule the airflow fraction, or set a DesignSpecificationOutdoorAir object for use in demand control ventilation. In order to make sure that the AirLoopHVAC maintains 100% outdoor air, set the Controller:OutdoorAir Mininimum Limit Type to "Proportional Minimum" and the Minimum Fraction of Outdoor Air Schedule Name to a schedule that is always 1.