It seems that my automated importing of downloaded music into the Music app needs to be slowed down to allow time for the music to be unpacked.
This is not the first time that I've run into issues with how Hazel processes files and folders. Occasionally, it'll try to act on a file while it's still being downloaded or moved. At other times, things have broken down because it has tried to expand a lot of ZIP files in one go, and The Unarchiver froze up.
The good news is that the music in question was purchased from Bandcamp, so I can easily re-download it.
The bad news is that the Music app doesn't automatically copy the replacement files into my library, and is taking an age to consolidate everything.
Some days, I'm literally too clever for my own good.