Keeping mouth open

[SarcasticDave94]

Your tongue is sealing the mouth off from the airway, as is normally happening while awake.

[G. Szabo]

I read about trying to keep your mouth shut when using a nasal mask to stop leakage but I dont get any leakage.
Why

Your sof plate is probably overly developed, and your tongue is situated so backward that they block entirely air leakage via your mouth when you pressurize your nasal cavity. Most people have the opposite problem.

[Made]

Your sof plate is probably overly developed, and your tongue is situated so backward that they block entirely air leakage via your mouth when you pressurize your nasal cavity. Most people have the opposite problem.

I think that even though I use a full face mask I am nasal breathing because of this.