I have just been on Live Chat with Netflix and I have a rsolution.
It sounds wacky ... but it worked ...........
Find an 18+ movie or show - I chose Rocco
Change the sound preference from 5.1 to English
This will set the default for ALL shows from then on!
I tested with multiple shows then exiting Netflix and restarting .... it works
I'm late to this, but on most devices if you go to the audio settings on that device (for most of my clients it is firestick/fireTV) you can just turn off Dolby Plus audio out and it will only push stereo audio.