Apple TV (4K) is outputting sound to (non-existent) rear surround channels when playing 7.1 LPCM audio on Sonos Arc.
The Arc should drop the extra 2 channels, or mix them into 5.1 output.
I’ve tested with movies on the Apple TV app and channel check files in Infuse on my LG B9 (eARC). This is persistent with all Multichannel PCM settings combinations.