I just bought both a TCL Roku 65" TV and an LG SL4Y Soundbar to listen with. The only connectivity options available is digital optical in/out which I have connected. There seems to be no sign of connection between these 2 devices even though it is clearly connected. The only sign of connection is when I volume up on my TV, the soundbar volumes down. Very confused and would like some answers because this feels a bit dumb when they're both connected.
S/PDIF doesn't offer the ability to allow your TV to control the audio level on a connected audio device. This functionality is only available when using HDMI-ARC. It sounds like the soundbar is somehow picking up IR signals from your TV's remote, and causing unintended side effects.
One way you can test this would be to install the free Roku mobile app, and connect it to your Roku TV. Use the mobile app to turn the TV's volume up/down (which uses a wi-fi based signal, and not IR). Do you see the same unintended behavior occur on the soundbar when you adjust the TV's volume using the mobile app?
I would recommend following up with LG support to report this issue occurring on their soundbar, and request additional help. You can reach them here: https://www.lg.com/us/support