Does an attic insulation baffle allow a rectangular undereave vent to have its intended net free area of airflow or is it reduced by the baffle?

Airflow does not only channel directly upward through the insulation baffle. Some of the airflow channels to nearby rafter bays.

Airflow does not only channel directly upward through the insulation baffle. Some of the airflow channels to nearby rafter bays. Thus, the total NFA of the vent is realized provided the insulation is not blocking the other bays.