If the compressor is working correctly the problem may be the “blend door” in the HVAC system. The blend door controls interior temperature by sending incoming air through either the heater core or the AC evaporator (or both) depending on the desired temperature.
If the blend door is vacuum controlled it could do exactly what you describe, since vacuum is high at idle and drops off as you open the throttle. At idle the blend door may start moving and direct airflow more and more towards the heater core. When you open the throttle the blend door moves back toward the AC evaporator.
A vacuum leak would explain this, assuming the system is vacuum operated.
Why was the expansion valve replaced?