I may be clutching at straws here but when i did the cabin pollen filter in my b-max i was advised to double check that the filter was inserted the correct way. The filter has an arrow embossed on one side of it which has to point in the direction of airflow into the cabin. Like i say, not sure if it would cause the issue you have but maybe worth checking ?
1 litre Zetec Turbo petrol 74Kw / 100 Ps. as in my avatar.