If Fenestron stall was real and was a function of too much or too little airflow through the Fenestron, it would be simple to reproduce it by hovering crosswind in a strong wind - it doesn't happen.
Not sure about air only flowing one way through the Fenestron either - max sideways demo on 365 provided a noticeable kick and change in pedal position as you accelerated and decelerated moving right as the airflow switched direction.
Fenestrons do use a lot of power but I really don't believe they stall - I've mishandled enough of them and never had a problem correcting yaw.