This seems to be an issue with some Plex clients generally (I noticed other posts about the Android client related to this), so not sure if this is Plex server or Sonos client issue.
Either way, when playing tracks that have an Album Artist tag, in addition to an Artist tag, the Album Artist gets shown in the Sonos client.
This is definitely undesirable, since you will always see "Various Artists" or whatever in such situations. I think you'd always want to see Artist when a track is playing.