这是Apple Music当前API中的一个bug。正如你提到的，不在用户曲库中但从AM流媒体播放的歌曲将不会返回专辑插图。
你应该在www.example.com上提交一个功能增强bugreport.apple.com（你也可以复制我6个月前的25413082）。
在此期间，您最好的选择是使用iTunes API来检索专辑插图，不幸的是，这不会返回正确的结果100%的时间...

我为此纠结了很长时间。如果这真的是一个bug，我不确定，但是你不能像你想象的那样把UIImageView设置为nowPlayingItem。你必须从Apple Music服务API中提取图像。参见文档。也可以看看苹果提供的here示例项目，以获得完整的理解。此外，here是我如何克服这个问题的完整示例。

func updateUserInterface() {
    if musicPlayerManager.musicPlayerController.playbackState == .playing { //Check if anything is playing
        if let currentItem = musicPlayerManager.musicPlayerController.nowPlayingItem {
            songAlbumLabel.text = currentItem.albumTitle
            songTitleLabel.text = currentItem.title
            songArtistNameLabel.text = currentItem.artist
            let albumTitle = currentItem.albumTitle  //Changes here!
            let artist = currentItem.artist  //Changes here!
            if let artwork = musicPlayerManager.musicPlayerController.nowPlayingItem?.artwork, let image = artwork.image(at: artworkImageView.frame.size) { // Check for local image
                print("using local image")
                artworkImageView.image = image 
        } else { 
            guard let developerToken = appleMusicManager.fetchDeveloperToken() else {print("oops");return}
            let searchTypes = "songs"
            var searchURLComponents = URLComponents()
            searchURLComponents.scheme = "https"
            searchURLComponents.host = "api.music.apple.com"
            searchURLComponents.path = "/v1/catalog/"
            searchURLComponents.path += "\(authorizationManager.cloudServiceStoreFrontCountryCode)"
            searchURLComponents.path += "/search"
            let expectedArtist = artist?.replacingOccurrences(of: " ", with: "+")  //Changes here!
            let artistExpected = expectedArtist?.replacingOccurrences(of: "&", with: "")  //Changes here!
            let expectingArtist = artistExpected?.replacingOccurrences(of: "++", with: "+")  //Changes here!
            let expectedAlbum = albumTitle?.replacingOccurrences(of: " ", with: "+")  //Changes here!
            searchURLComponents.queryItems = [
                    URLQueryItem(name: "term", value: (expectingArtist! + "-" + expectedAlbum!)),
                    URLQueryItem(name: "types", value: searchTypes)
                ]  //Changes here!
            var request = URLRequest(url: searchURLComponents.url!)
            request.httpMethod = "GET"
            request.addValue("Bearer \(developerToken)", forHTTPHeaderField: "Authorization")
            let dataTask = URLSession.shared.dataTask(with: request) {
                (data, response, error) in
                print(response!)
                if let searchData = data {
                    guard let results = try? self.appleMusicManager.processMediaItemSections(searchData) else { return}
                    self.mediaItem = results
                    let album = self.mediaItem[0][0]
                    let albumArtworkURL = album.artwork.imageUrl(self.artworkImageView.frame.size)
                    if let image = self.imageManager.cachedImage(url: albumArtworkURL) {
                        print("using cached image")
                        self.artworkImageView.image = image
                    }
                    else {
                        self.imageManager.fetchImage(url: albumArtworkURL) {(image) in
                            print("fetching image")
                            self.artworkImageView.image = image
                        }
                    }
                }
            }
            dataTask.resume()
        }
    } else { // Need to set it to some kind of default values
        artworkImageView.image = nil
        songArtistNameLabel.text = " "
        songTitleLabel.text = " "
        songAlbumLabel.text = " "
        }
    }
}

MediaItem类中有一个名为MPMediaItemPropertyArtwork的属性。您可以通过以下方式检索相册的图稿：

MPMediaItemArtwork *artwork = [mediaItem valueForProperty:MPMediaItemPropertyArtwork];
UIImage *albumImage = [itemArtwork imageWithSize:cgSizeYouWant];

我发现最好的方法是使用从目录中获取歌曲的Apple Music API请求documented here。
你只需要传入歌曲标识符（用于播放的标识符）和用于播放歌曲的店面标识符，这样总是能得到准确的结果，而应答器使用的搜索请求可能会失败。

有了SwiftUI，一切变得超级简单-

if let artwork = mediaItem.artwork {
     ArtworkImage(artwork, width: 85) // or whatever width you need. 
         .cornerRadius(10)
}