What was this a playlist of? Your own tracks stored on your Mac? Tracks streaming from Apple Music? Tracks that may have been offloaded for later re-download/streaming by iTunes Match? Or something else?
My music collection is 100% local tracks - either ripped from external media or purchased. Nothing has ever been streamed. Nothing has ever been sync’ed from any cloud service. And none of my playlists have ever spontaneously changed.
How did you choose the playlist? Is it possible that the name of your playlist matches the name of a playlist on the Apple Music service, so that rather than starting a playlist from your library, you were starting a playlist with the same name from Apple Music (the service?)
Not to belabor, I suppose, … I doubt it was a pre-configured Apple playlist
Specifically called the playlist: AAA-Rowing, so it’s at the top of my playlist queue… about half tracks from my downloaded library, and half tracks pulled from Apple Music, and/or tracks I own but maybe still in the cloud. It’s hard to determine whether others I do not own were downloaded in the Apple Music folder created by Apple. I think maybe Apple Music downloads a temp when you add a track via subscription access