Streaming Apps Not Playing Through HDMI eARC

I have a new LG smart TV with a Sonos sound bar connected via HDMI eARC and two wireless Sonos surround speakers.  Sound from the Xfinity TV channels is great, but when choosing any of the streaming apps for content, the audio playback is terrible.

Watching Thursday Night Football last night on Amazon Prime, I noticed that I was only getting sound from the soundbar, but it was monotone.  I switched from the Xfinity Prime app to the TV's Prime app, selecting the same program, and the sound was back to surround stereo through all speakers.

Is  there a setting or connection that I'm missing?