Sync related problem: some music files fail to sync

Below is the issue in short:

According to iMazing, some of the music tracks could not be transferred to the iPhone because they are in the cloud. However, these music files are not in the cloud. My iCloud is empty. I double-checked that.

I can exclude that these music files in question are faulty or corrupted. They work fine in the iMac Music app. The files include AAC, MP3, and Matched AAC music files.

What could be another reason why these files do not sync?

Below is the issue in more detail:

I removed all music files on my iPhone by disabling Music sync on the Mac. Then I enabled the Music sync again. I synced one playlist with about 47,000 tracks. Among these tracks, 16 were not transferred. iMazing tells me they are in the cloud, which they are not.

I looked for each of the 16 tracks on my iPhone. I found them. Each entry disappeared when I clicked on it. I conclude while the Music track DB file on the iPhone listed these 16 files, the actual music files were not transferred.

Subsequently, I connected the iPhone to the iMac again and looked in the Music app under Devices/iPhone for the 16 tracks in question. All 16 tracks are listed there. In iMazing on the other hand the 16 tracks disappeared from the list.

If I sync again nothing is transferred even though 16 tracks are missing. This can be explained by the macOS music track DB list still containing the 16 missing files. 

To rule out that this was just a single freak event I repeated the whole procedure starting with removing all music from the iPhone. The result was exactly the same. 

Again. What could be another reason why these files do not sync?

Does anybody know a trick to overcome this issue?