Music disappears from Apple Music library.

In order to solve a problem where AMPLibraryAgent kept spiraling out of control and using the entire CPU, yesterday I created a new library file and reimported all 2 TB or so of my downloaded music. At least I had access to it without my computer restarting every 20 minutes, right?

Then today, because "Recently Added" was blank (a recurring bug with this buggiest of all applications), I quit and restarted Apple Music. And just like that, presto! Anything I had downloaded before this morning was missing from the library.

I am currently reimporting (again) my entire collection of digitally downloaded music. I can't for the life of me figure out why this happened; if anyone has heard of this problem before and what might cause it, I'd love to know. Additionally, if you have a recommendation for a music player/library manager that, as someone used to say, "just works"--I'm all ears.