Apple today released iTunes 12.4.2 to fix a bug where short Apple Music tracks caused playback problems. Playing a very short track would prevent the following tracks from playing.
Download the update from the Mac App Store, or from Apple’s website. (At the time of this writing, the new version isn’t yet on the iTunes Download page; it should be there soon.)
This 12.4.2 update seems to have once again fixed sync issues I had with my 6 Plus using the previous version (12.4.1.6).
This 12.4.2 update seems to have once again fixed sync issues I had with my 6 Plus using the previous version (12.4.1.6).