Someone on Reddit is also experiencing the same issue. I haven't had a chance to look into it more. It's likely that MusicKit JS isn't initializing for an unknown reason (my app initializes after MusicKit JS).
For some reason the HTTP request for music.js returns no content. I tried with curl and got the same response, so it doesn't seem to be browser related.
The curl request does work when I do it on my VPS, so I'm guessing it's something to do with Apple's CDN.
The curl request does work when I do it on my VPS, so I'm guessing it's something to do with Apple's CDN.