I might have discovered something – I am going to describe what I did first, and then say what I think it means, regarding if and when Apple adds the Album ID and Artist ID tags:
I have several tracks from “When a Man Loves a Woman”. I matched them via Picard with this release. Some of the tracks have iCloud status “Matched”. Rather than downloading m4a versions to replace them with, I first created copies of the audio files and imported them into iTunes.
In Tunes, I waited for the imported set to become matched again with iCloud Music Library.
Now I had two sets of matched tracks - the original set and the imported set.
Then I replaced the imported set by downloading the m4a versions. Then I checked with Kid3 whether Album ID and Artist ID tags are present. They are not.
Then I replaced the original set by downloading the m4a versions. This time, Album ID and Artist ID tags present.
So, this is what I think this means:
I signed up for the iTunes Match service 11 months ago. This means my library was scanned 11 month ago and the “iCloud Status” information on all of the tracks is from that time.
When I replace one of the tracks that were matched 11 months ago, Album ID and Artist ID are added. When I replace any track that was matched today (or more recently), that does not happen.
That means the scheme (for lack of a better word) that Apple uses to describe the link between a customer’s local copy and a track in iCloud Music Library has changed. They are no longer writing Album ID and Artist ID into the files.
However, the information that instructs what to write into the files, does still exist. So, when I request a download today for a track that was matched some time ago, the information from some time ago is used and that’s why I get Album ID and Artist ID for this track.
I will continue to test this theory with the tracks I am going to process next.