Only your newer tech will say it works all the time so always have the optical link handy.
Netflix wrote the app so ask them about the app specifics.
I think you have to get heavy handed with the makers on this one. They don't want to sit down and discuss why this happens and I can't reply to every post and repeat the prior discussion such as
You have the best hookup but have to fish around the netflix app to see if it has some setting about audio.