They haven't. I left Apple Music but just came back again and decided to try things a little differently.
I created a new, empty iTunes Library. I made that the library I sync with Apple Music. There's no point syncing certain albums I own when they already have them in Apple Music. If there's something missing in Apple Music that I own and that I want, I drag the files from my main iTunes library to the newly created empty one. I only do this one album at a time, and I only drag the MP3 files, not the enclosing folder. I seem to have way less artwork matching problems this way. Not sure why.
I now have a huge Apple Music library with no redundancy, no mismatched artwork, and no mismatched songs.
As a result, I'm enjoying Apple Music MUCH more this second go-around.