I have a similar setup with my TV connected via optical audio to my Yamaha receiver. Selecting the source on the receiver shouldn't change the TV source. If your TV is somehow switching automatically to an HDMI or AV source, then you can just manually switch it back to the TV's Netflix app.
On my Yamaha, the optical source is AV1. Do you have something else connected to the AV source in addition to the optical audio? If so, move it to a different one.
I have a Yamaha RX-V665 receiver and my cable box and Blu-ray player feed it via HDMI cables. Then I ouput the receiver to my Smart TV via HDMI. When I switch sources on the receiver, the sound and picture switch together. Easy enough.
But here's my problem: When I watch Netflix via the app built into the TV, how do I output the audio from the TV back to the reciever so I can enjoy the 5.1 soundtrack of the movie? I tried using the optical out of the TV to the V665 but when I select that source, the picture changes to that source has well. Is there a way to do what I'm trying to do?