There are also vacuum pods used to control various doors in the ductwork. A leak in a pod diaphragm can also cause a problem like this. Some of those pods are easy to access, others are not.
I’m not real familiar with the layout on this particular model but with many Fords the pod that controls this function is often behind the glove box. This makes it one of the easier ones to access; if needed.
This is where the diagnostic footwork starts but I agree with checking the one-way valve first. A vacuum gauge/vacuum hand pump can also help to some extent in tracking these things down.