You are using the TV's own Netflix App, there can be a menu in the Netflix App to select what audio system to use. Netflix changed over time so I can't tell you the exact menu location.
I have a new LG 65" OLED tv, and a new Yamaha AR receiver RX-V685, with a 5.1.2 speaker configuration. I get fine sound watching TV, and listening to music. When I turn on Netflix, the only way I can get sound is thru the TV speakers. i am using new HDMI arc cables. Any ideas?