iTunes on Macbook disconnects from iCloud Music Library, songs disappear
I added some music from other sources to my iTunes library on my MacBookPro (mid-2012). I have an iTunes Match subscription and the songs were added to my iCloud Library, and I can access them on my other devices (iPhone, iPad).
Now the problem: After I close out of iTunes on my MBP, the next time I open it, the songs I added recently (not all, just from the last few months) have vanished. When I go to Preferences, I see that the "iCloud Music Library" box is no longer checked. When I check the box again, iTunes connects with iCloud again, and sees the songs in iCloud only, but not the copies that are already physically stored on my MBP. I can download them again, but then I have two copies of each song stored physically on my MBP. And when I close out of iTunes, the next time I open, the whole thing goes back to the beginning (songs not in library, iTunes disconnected from iCloud Music Library).
I had the same problem several years ago, it went away with an update. But now the problem is back, and has persisted through several updates.
I have the latest update of both iTunes (12.7.3.46) and MacOS High Sierra (10.13.3).
MacBook Pro, macOS High Sierra (10.13.3)