It may just be that it is next to impossible to really re-program your autonomous breathing. When your mind is focussed on something other than breathing, your body just does what it needs to do to get some air.
It may also be that your body will switch to nose breathing "automatically" when you're wearing the mouth positioner. Just like a nose breather doesn't die of asphyxiation when his nose is blocked with mucus - the human body is fairly smart about staying alive.
Jim.