I'm a premium user on iOS 12 and Windows 10, syncing local tracks on the same wifi network.

I have a folder of local music. Regularly, Spotify de-syncs previously synced local playlists from my iPhone, always for songs that aren't on Spotify. They can never be re-synced. 

Particularly common are anything by Beyonce or Jay-Z, period point blank. It has also happened with with Michelle Shocked, Joanna Newsom, and several others. I own all this music.

I've had to start appending something to the album name (and sometimes both the album name and the song title and the file name) to get the tracks to resync to my phone correctly. That is absurd, since it messes up my song tags. 

How can I prevent this from happening? I'm not going to use Spotify if it doesn't respect my legally owned, local music. Why would I use a service that won't let me play what I own? 

Please don't recommend I uninstall or reinstall anything. I've tried that. 

Thanks.