That's not
quite correct, as I've found out over the last few days of bashing my head against a wall with this.
Apple Music
matches your music by metadata alone, whereas iTunes Match uses acoustic fingerprinting. This has resulted in most of my live / bootleg albums having their tracks replaced by the studio versions. The end result for me is that Apple Music as-is is unusable - if I don't use iCloud Music Library then I can't use Apple Music content offline, if I do then it mangles my library. I've so far not pulled the trigger and paid for iTunes Match to see if I can resolve the issue.
Obviously the other difference is iTunes Match gives you DRM-free versions of 'your' music on other devices whereas Apple Music will give you DRM versions that stop working when you stop paying the subscription. That distinction doesn't bother me, as I have all the DRM-free versions (and CDs they originated from).
In reference to your question - no idea. Sounds like something is broken as you should be nowhere near the limit. Mine uploaded about 2,000 songs when it mangled my library, which is likely more than your 15 albums. I'd suggest contacting their support.