Does Apple Music affect syncing my iPhone 12 to iTunes on my Windows 10 PC?

See Identify cloud status icons in your music library on your Mac or PC - Apple Support. Typical issues are tracks that are too long, or have a low bitrate. Depending on what this content is you could potentially change the media kind to Podcast or Audiobook to allow you to sync it from your computer without turning off the Sync Library feature of Apple Music.

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You can download Apple Music tracks to your computer and/or your devices. Anything you don't download can be streamed. You no longer music directly over USB, but other content can be added that way.

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Typically if you have Apple Music you stop syncing directly and allow changes in your iCloud Music Library to sync automatically over Wi-Fi.

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