I've had this vehicle since new, the switch for external or internal airflow located on the lower RHS of the heater panel has always switched the function between the two modes.
Now, randomly, the airflow flap is switching between the two modes without applying the actual switch. Mainly during hotter weather and usually it locks up in the mode for "internal" air circulation. The normal illumination indication in the panel does not light-up when it does this.
The handbook states that internal air circulation should not be applied as a full-time measure during operation of the aircon, thus I imagine it is only supposed to be used to prevent outside foul air entering the vehicle or for very humid weather conditions.