After some experimentation, I think night mode might be causing this issue. I turned off night mode, and suddenly stereo PCM content sounded way better.

My current hypothesis is that when night mode raises the level of quieter sounds, it accidentally raises the noise floor and emphasizes lossy codec artifacts that would typically be inaudible. I think the issue doesn’t present with Dolby surround formats as much because those tend to be a much higher bit rate, both per sample and per second, so adjusting the level doesn’t emphasize quantization noise or lossy artifacts as much.

The source audio was a YouTube video using lossy Opus audio, so that’s probably where the lossy artifacts came from, and they were just being overemphasized by night mode.

When playing stereo content, the Beam will perform an audio up-mix of the stereo signal to create a simulated surround sound. This means you’ll still hear most audio out of the Beam, but the surround speakers will play what is determined to be ambient audio. The quality of the audio from the surrounds is largely dependent on the quality of the stereo mix. A lot of YouTube videos have pretty low quality stereo audio, so the ambient audio played from the surrounds won’t sound very good and sometimes won’t play at all.

In fact, a lot of YouTube videos that claim to be in a higher codec like Dolby Digital or Atmos are actually in plain stereo. So they won’t play at all on the surrounds, as @GuitarSuperstar was saying. 

Right, I’m aware that Sonos upmixes stereo content and sends some of it to the surround speakers. My question was whether it was possible to disable that behavior, or always send stereo content at full strength to the surrounds, instead of what it currently does, which is to selectively send what it (sometimes incorrectly) interprets as ambient sound.

After more experimentation, it seems that this issue doesn’t fully go away without night mode, it’s just less noticeable. But it’s still very much happening.

So no, this question is not answered, and I’m not sure who marked it as answered. 

No, it is not possible to disable that feature, and you cannot send full stereo content to the surrounds for TV sources. 

Sure, and some actually are. But what I’m talking about is Sonos upmixing confirmed stereo 2.0 content and sending some of it to the surrounds.

Regardless of the format you can’t send a copy of the front channel signals sourced from the TV directly to the surrounds.

That option is only available for sources streamed directly to the Sonos.