For clarification, I think what you are referring to was that the files are not loading inside of Music.app. Your files are still on your machine.My Music library has been deleted with this build.
Check release notes:
Music
Known Issues
- Navigating using the back button might produce unexpected results. (51248128)
- Music might take a long time to launch for some users. (51207753)
- You might be unable to scroll content on shelves. (51248128)
- Artwork changes might not immediately appear; playlists will be added in an upcoming beta. (51201715)
- Music might not remember the previously selected tab when it's relaunched. (50922604)