I made the following change and it may have fixed this. I noticed that the Sonos 5 and Sonos One SL appeared twice in Settings->Audio, once “via Airplay” and once “via Sonos Streaming”. I disabled the Airplay versions, leaving only the “via Sonos Streaming” versions.
Now everything appears as capable of being grouped. I have to pause the music before adding/removing to/from a zone or Roon loses control of the audio device and stops playing. But, all my zones show to be group-able now.
Was anything changed on the backend, or is this the fix? (But not the complete fix, since Roon is still losing control of the audio devices.)