Music scan not working properly

I purchased a bunch of CD's and were adding these as I was ripping them.
For example I successfully added Lady Gaga - A Star Is Born Soundtrack, played it while ripping other, but later noticed music is now missing.
Instead of scanning for new files, I did a complete rescan, which completed successfully, but music is missing.

For example Lady Gaga - A Star Is Born Soundtrack - only contains:
track 5 + 17 + 20 + 22 + 25 + 27 + 28 + 30 + 34 (9 in total)
On the harddrive, all 34 tracks are there (FLAC).

Logitech Media Server Version: 8.1.1 - 1610364020 @ Thu Jan 14 06:27:19 CET 2021
On a Synology DS415+ (not running LMS in a Docker container)

Library Statistics
Total Images: 819
Total Videos: 1
Total Tracks: 8,835
Total Albums: 715
Total Artists: 973
Total Genres: 102
Total Playlists: 13
Total Playing Time: 573:12:33

I'm mystified ...

Best regards,
Claus