OK - I decided to do something radical last night. I followed the steps in the support article I pointed out above (https://support.apple.com/en-us/HT204964).
Short story: it didn't help with adding missing album covers to iTunes Match in iCloud Music.
Long Story: After following the steps above, and just to make sure, I didn't just reset my iCloud Music library, I also removed ALL my music from my iTunes library and started with a clean slate so-to-speak — that's not a biggie for me, because I'm an album guy (otherwise known in some circles these days as "boring") — in short, after following the steps above, I opened iTunes, did the "restore" step they talk about (during which I was signed out from my iTunes account) and then exited iTunes and removed all the contents from the iTunes folder, as if it were a new computer.
I opened iTunes to restore the default folders and files, and then I quit again.
Then, I added all folders of my music (1054 albums) back to the Automatically Add to iTunes folder; after that, I had to "restore" some album covers. Then I signed into my iTunes account. iTunes asked me if I wanted to sign in and add my music to iCloud Music (clearly, after signing in, it recognized me as an iTunes Match subscriber and recognized my "new" library). I choose yes — and then it did its "nontransparent magic thing" (which was a lot faster this time around). Mind you: at this point, my library was complete with covers, and everything in proper order. When iTunes Match was finished, I went to my MBP and iOS devices — both had empty libraries — and signed into iCloud Music. A short time went by, as the library was loading on each device — and once the library was up: boom — same album covers as before were missing, even though these albums have covers in my "master" library.
One other thing I noticed during this little time-wasting exercise: This time around, iTunes Match UPLOADED several matched AAC files that I had previously downloaded to replace lower quality songs. Three months later, it no longer matches previously matched songs from albums still available in iTunes?
I wish it would just work — as we were promised (before the June update and the run-up to Apple Music, which I'm not subscribing to, my covers were all present in the cloud ).