Where are these playlists ‘created’? If it’s not a Sonos playlist, Sonos only has ‘read’ access, it can’t change a playlist created elsewhere. 

I suspect it’s where you’re streaming from that is changing your playlist with other stuff. 

The playlists are created in the Sonos app, and streamed from a hard drive onto which I’ve burned CDs.

Original playlist tracks are literally replaced by tracks from other artists/albums—usually, the new tracks will come from one random album. It’s bizarre, and really irritating. 

I’d agree on both counts, and don’t quite understand how Sonos itself could be doing this, there just isn’t any ‘process’ in place that I’m aware of. 

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

And I’d request, if you’re willing, that you share what you find out here. Sonos ‘substitutions’ would be extremely troubling.