I am also facing the same loader issue with Apple Music authentication using MusicKit. It was working with Apple Music version 6.5.1, but after updating to version 6.5.2, it is not working again. It just keeps loading, and nothing happens.

@joneswr is right. I am facing same issue. its works with old apple music app version 5.2.1 but not with new version 6.5.0.