I am no expert in this area.
But, I believe this may be a compensatory articulation error, just like a cleft palate learned behavior.
I have worked with one child who has a tendency for hypernasal speech, she has Beckwith Weiderman syndrome. We have used a simple swimming nose pinch/plug to help her produce sounds with an oral airstream. I also told her about making sounds at the front of her mouth, not the back of her throat.
Hope you get the help you need.