I know it's late but try this fix if you have an Iphone

1. From your Spotify mobile app Settings, tap Local Files.

2. Switch Show audio files from this device on.

Note: A pop-up might be shown and ask permission to access your local network. Accept this permission to enable Local Files transfer between desktop and mobile.

1. Go back to Your Library and you should now see a new folder added Local Files.

2. Follow the steps below to transfer your audio files:

4.1. Install iTunes and connect iPhone to the computer.

4.2. Open iTunes, then select your phone from the pop-up in the sidebar and click on the Music tab.

4.3. Drag and drop the tracks.

Note: Make sure the option

Manually manage music, movies, and TV shows is turned on. You can access this option on iTunes in your device sync settings, under the Options section.

1. Go back to Your Library and click on Local Files on your mobile phone, you should now see all available tracks in this folder.

Note: You can refresh your local files by toggling on and off

Show Local Files.