Question 5.1 to 2.0 optical audio error

[Jakubdante]

Hi I’m watching Apple TV+ series on my LG 50UM7450PLA with optical audio output and those videos have one issue. Everything is fine until a song comes in the background. Then the vocal is heard only in right channel. My optical output is connected to Yamaha amplifier CD- NT670D. All other videos from Netflix, prime and even other movies available on Apple TV work fine. Only Apple TV + productions have this issue. Actors’ voices are fine, music is fine, foyer is fine. Only songs have don’t have a stereo sound. Help.