I think the cause is the same problem reported here. The comment (by me) about the problem being fixed is erroneous - it appears this only fixes itself after Plex’s scheduled tasks have run and calculated the track bandwidth. I think when I tried it I must have coincidentally hit that scheduled task so it appeared fixed.
Given that the tracks play after the scheduled tasks have run, but not before, makes me think this is a Plex issue, as it is not providing the info required by Sonos to play the track. I don’t know if this is badly documented in Sonos’ API.
I think the fix is just for Plex to calculate the bandwidth on library import, not just as a scheduled task - the code is obviously in there, but just not triggered. Feels like a one-line fix to me, but they’ve never addressed this 
