Unable to sync local files to iPhone if file is not licensed to Spotify.

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Hey folks, 

I am able to sync local files from PC to iPhone using Offline Playlists but only when the included files are part of the licensed Spotify library. Titles for the unsynced songs do appear on the phone but I get the "Song Not Downloaded" error. Songs that have a license download fine.

Any known solutions? Would love to be able to use songs from CDs and other downloads that I own but Spotify does not have rights to.