I am also having the same issue. Just noticed this last night watching Avengers and was pretty upset. Must be an app limitation and hopefully will be addressed with an update later as there is no setting anywhere that changes this. As you stated Netflix works fine with 5.1 so this is an app specific issue. 😕
Welcome to the club! Seems a lot of us are having this issue (86UH9500 2016 model), see the other Disney+ thread if you want to keep track. I would also suggest contact Disney+ on their support page and let them know the issues you are having.