That’s weird indeed. Do you have a 5.1 system that you can test it on?
Yeah and it’s indeed weird 'cause all channels display something. Unrelated questiion: you’ve chosen 448 kbps for the audio export, why not 640 kbps which could provide a deeper quality ?
edit: by the way, do you have an external chapter txt. file ? I cannot use iso format so I convert m2ts format into mkv hence I lose the chapters of the bluray version.