Apple to send out update this week to help prevent iTunes bug from deleting more music from users' libraries29
Apple says that it will be sending out an update to iTunes early this coming week which takes "additional safeguards." The company believes that an iTunes bug could be to blame, hence next week's update. If your music is disappearing from iTunes, Apple suggests contacting AppleCare.
Apple, with its theory that a software bug is at fault, is basically dismissing talk that Apple Music's problematic algorithm is the issue. Some were saying that Apple Music was deleting files when trying to match its streaming library to songs purchased by an iTunes user, and kept on the hard drive of his/her original Mac. This matching is done to make sure a high quality version of a song is streamed when a user is listening to tunes on his/her iPhone, iPad or a different Mac.
Another possibility, covered by Apple on a support page, has to deal with the disappearance of music, video and other content after updating iTunes. If that explains your situation, you might want to click on the sourcelink and try Apple's suggestions.
The good news is that Apple, while unable to replicate the issue as we noted, is aware of the problem. Hopefully this upcoming update closes the black hole that has been devouring iTunes subscribers' songs.
source: iMore via PCMag