If Your Album Artwork Not Showing Up in the New Music App on macOS Catalina after You Imported Everything
Just delete the three sqlite database files inside ~/Library/Containers/com.apple.AMPArtworkAgent/Data/Documents/ (so that Music.app will rebuilt the artwork indexes database for you) and you are good to go.
Maybe not, because before that, I also copied everything inside ~/Library/Containers/com.apple.AMPArtworkAgent/Data/Documents/artwork/ from the old machine to the new one. The sqlite database files are presumably responsible for maintaining the index between the artwork image files and the music files. Deleting them seems to be the only way I know to trigger the update after the manual import.
By the way, I can’t confirm if this folder is only used by Music.app or by iTunes.app (before Catalina) as well, because my both Macs are running Catalina Public Beta (that’s a stupid move, I know). However, when I sorted the files by date added, the oldest one was created at the time when I upgraded to Catalina so it looks like to be a Music.app-only thing.
Actually, I guess if you don’t have the ~/Music/Music/ folder (which only exists after you upgraded to Catalina) beside the old ~/Music/iTunes/ folder, the album artworks should be generated automatically during the upgrade automatically, because that’s how my old Mac works. If you already have ~/Music/Music/ folder then you must have a previously imported (from iTunes.app to Music.app) library and your artworks should be either on your current or previous computer.
Don’t forget to prepare the backups though. I didn’t research on what’s under the hood of the new Music.app, but due to the fact that this library is your only one on macOS Catalina at this time, using option + click to switch between libraries won’t work here. Besides, the new presumably proprietary .musicdb db format is replacing the good ol’Library.xml and .itl files in iTunes, so please pay attention when poking around if you care about the integraty of the metadata or the music files in the library itself.
Anyway, that might or might not explain why copying the Library (~/Music/Music/ as well as ~/Music/iTunes) from a Mac to another results in missing album artworks.
The way I found it out is pretty straightforward. I used fswatch to locate what files changed when a song was added to the new Music.app library, and if you tinker around with the relavant files you found in the previous step, it won’t take you too much time to glance at how the new Music.app local database might work.
All in all, this might only apply to my case (all relavant files/folders were copied over but artworks are still not showing in the app) but it should fit into similar situations where involves clean install of macOS Catalina (and I used manual copy instead of Time Machine to transfer the library). I couldn’t find any practical solution on the Internet before I figured it out on my own but there are some other people have the same problem. Hope this post can help those who encountered similar issues.