Apple music sync concerns/issues

I upgraded to Catalina almost 2 years ago…I have a 2015 Macbook Pro and am now -as of 2 weeks ago- running Big Sur.

After upgrading to Catalina, I have an issue with syncing my music to my iPhone SE 2020. Whenever I add a new playlist and wish to sync and add just that playlist, inevitably my entire music library syncs and I cannot, for the life of me, figure out what’s going on. I had hoped that upgrading to Big Sur would address or fix what I assumed was a bug or glitch. When running Mojave only that new playlist or album would sync or be added.

Prior to the Catalina upgrade this was never an issue or concern.

Any insight or idea as to what I might be doing or missing would be appreciated.

What happened between Mojave and then Catalina and now Big Sur that might cause this.