Plex not reflecting the metadata from the audio files

[bonnyfused]

@OttoKerner

Is this one collaboration album stored within the same subfolder as all the Lucio Dalla solo albums?

No, it’s not:

With collaboration albums in Plex you have to decide on one artist name to put into the AlbumArtist meta tag.
You can put both names into the [track] artist tag, so that they at least are repesented when played back. But the pairing won’t appear in the list of the labum artists in Plex.

I just applied this change: I set the “Album Artist” to “Lucio Dalla” and left the “Artist” for each track to “Lucio Dalla & Gianni Morandi”. After updating the meta-tags in Plex, it is still wrong.
This is MusicBrainz Picard:

I refreshed Plex and the red marked part appeared along each track (wasn’t there before):

If I edit a track, it still ignores the meta tag:

What is disturbing further, is the fact that now track 01 has turned to track 00 - even though you can clearly see in the first screenshot that the track number is 1.
What is Plex doing here???