Matching songs is hard, and Apple Music's not doing too well with it. But you can keep it from messing up your library pretty easily.
Earlier this week, Kirk McElhearn posted a rather-worrying article about iCloud Music Library's "matching" algorithms.
As you might know, Apple offers the iCloud Music Library service as part of Apple Music and iTunes Match. This scans your Mac's music library and attempts to do two things: "match" tracks in your library with songs in the Apple Music or iTunes Store catalog (which catalog depends on which service you're subscribed to), and upload songs it can't match directly to iCloud.
If you've used iTunes Match in the past, you may know that it matches music using acoustic fingerprinting, which means that iTunes scans the music, and matches it to the same music. It doesn't matter what tags files have: you could have, say, a Grateful Dead song labeled as a song by 50 Cent, and iTunes Match will match the Grateful Dead song correctly...
Apple Music, however, works differently. It does not use the more onerous (in time and processing power) acoustic fingerprinting technique, but simply uses the tags your files contain. And it can lead to errors.
This means that by changing metadata on a track, you may be able to "fool" Apple Music into matching it with a different track in your iCloud Music Library.
Does this suck? Yep. It's also likely a bug, and I have no doubt that the folks at Apple are well aware of it and working hard to make sure it happens as infrequently as possible—preferably not at all.
I can't reproduce it in my iTunes Match/AM hybrid library, or my boyfriend's Apple Music-only library
I have both an iTunes Match and Apple Music subscription, and decided to duplicate McElhearn's testing to see if I could get the same results. Answer: Not really.
I used my auxiliary MacBook Pro which has a handful of local songs; most are stored in iCloud Music Library, matched with my desktop iMac.
I did this test three times for both a matched and uploaded track: First, I saved a copy of an AC/DC track that iTunes Match had matched to my desktop and reuploaded it to iTunes as The Weeknd's "Can't Feel My Face"; upon local deletion and redownload, the track remained AC/DC's music, though it kept the erroneous metadata I'd assigned it.
For the uploaded track, I added a 7-minute voice test I did for The Incomparable Radio Theatre on the Air, and labeled it as Foreigner's Juke Box Hero. Interestingly, when I first uploaded the track to iCloud, it very briefly matched as Apple Music; when I deleted it from my hard drive, however, the track reverted to showing as "Uploaded" in my iTunes library, and upon redownload, played the same 7-minute test as before. On redownload I did get pretty Foreigner album art, however.
Update: I also ran these tests three times on my boyfriend's Apple Music-only library, with both an uploaded Incomparable track and an Apple Music-matched Billy Joel song (trying to transform it into a Weeknd song). No mis-matching.
What does this mean?
Likely what I've been saying about Apple Music, iCloud Music Library, and Match from the start: Matching tracks is hard, and if you're trying to trick a complicated system, or have unique tracks that Apple's never scanned before, you're likely not going to be too happy. Unfortunately, I suspect this is the nature of the matching game—doing this on such a large scale requires active user testing, and that means bugs and mistakes.
From my tests, it looks as though Apple is still using acoustic fingerprinting for iTunes Match accounts, but may be augmenting this with metadata matching for Apple Music. I wouldn't be surprised if, due to the whole "having to connect to the Internet" thing, Apple Music's metadata matching occasionally happens before iTunes Match's fingerprinting; if you happen to immediately delete your track as it's processing, you may wind up accidentally with an Apple Music track.
And yes: On this, Apple has failed. iCloud Photo Library was very smartly released as a beta, because of similar syncing issues and testing that needed to happen. iCloud Music Library wasn't, nor was it released with comprehensive documentation or even a warning to back up before upgrading. As a result, quite a few people have seen their home music libraries show up as bizarro copies on their auxiliary devices; those who don't have backups are even worse off.
That's devastating to both users and Apple. There are so many great things about iCloud Music Library and Apple Music, but right now, they're getting overwhelmed on social media and in the press by users having serious issues with their libraries and apps. Songs that mis-match, albums that won't download, buttons that don't work. Every time the service errors, people feel less-inclined to trust Apple with their cloud data, and consider alternatives that may be more stable, but not as secure. It's a shame.
I'm nervous now. Should I not use iCloud Music Library?
That largely depends on your library and what's in it. If you mostly have purchased tracks and ripped songs from studio albums, you should be fine with iCloud Music Library—but make a backup just in case. Remember: iCloud Music Library is, ultimately, making a secondary copy with its matching and uploading. You'll get these matched and uploaded copies when you download tracks on secondary devices, but it shouldn't mess with tracks local to your hard drive. And please: Do not use iCloud Music Library as your backup. It was never designed as a backup service.
Shouldn't and doesn't, however, are two different things, and as I said before—matching is hard. So if you have a history of problems with your Mac's iTunes library and you're concerned about iCloud Music Library messing up your tracks, it's simple: Just don't use it.
For those that are having problems—either matching or otherwise—turning off iCloud Music Library and restoring from a local backup of your music should bring everything back to normal. I made a really handy guide last week for people who want to use Apple Music without iCloud Music Library, which details a few different ways you can set up your devices to prevent your primary library from getting screwed up.
Make a backup. Turn off iCloud Music Library. Check our our troubleshooting guide. Call Apple. Or ping us in the comments if you're confused about this whole thing and this didn't help straighten it out.
Updated 2:07PM EDT to add tests on an AM-only library.