First let me state that this is not an issue with MusicKit, but I've raised a radar on this and have received 0 response, which is, sadly, kinda normal.
However, it's kind of an important issue and my app relies on this feature to work consistently, so I thought I'd mention it and that the amazing @JoeKun might have an idea how to escalate it.
Here's the problem in the iOS Music app:
- Add a song to an existing playlist
- Add another song to the same playlist
- Add another
- View the playlist
More often than not, one of the newly added songs is missing in the playlist to which the song was added.
There are entries in Recently Added for newly added songs that are not in the playlist where the songs were added.
- View the songs in the Music app on Mac.
- Query the playlist to get all songs via the Apple Music API.
The Music app on Mac and the API results align, yet differ from iOS Music app.
There are entries in Recently Added for newly added songs that are not in the playlist where the songs were added.
In summary, when adding a song to a playlist using the iOS Music app, it sometimes does not add the song.
For the record, I am awaiting the confirmation message each time a song is added.
Adding tracks via the macOS Music app, seems to always work. This issue only occurs with the iOS app.