Firstly i would like to start by saying that the new Premium Music Features are great and the music scanner is mostly accurate.
What I have noticed is that the music scanner assign tracks to the wrong disc when you have a multi-disc album were the same track name appears on more than one disc at the same position. In this situation both tracks get assigned to disc 1 and extreme situations where disc 2 has the same track listing as disc 1 (e.g. The Bravery: The Sun and the Moon Complete) you end up with a single disc album with duplicate tracks.
In these situations I had to manually edit the metadata in Plex to sort it out. The tags in the music files were correct but obviously ignored.
.
I'm also getting a problem where multi-disc albums are not being grouped by disc.
The tracks have valid ID3 tags and are named like so:
The Magnetic Fields/69 Love Songs/1-01 Absolutely Cuckoo.mp3
The Magnetic Fields/69 Love Songs/1-02 I Don't Believe in the Sun.mp3
...
The Magnetic Fields/69 Love Songs/2-01 Roses.mp3
The Magnetic Fields/69 Love Songs/2-02 Love Is Like Jazz.mp3
...
The Magnetic Fields/69 Love Songs/3-01 Underwear.mp3
The Magnetic Fields/69 Love Songs/3-02 It's a Crime.mp3
...

I have noticed that Plex also incorrectly sees song-numbers in multiple discs as duplicates. I have the Chess concept album, which is numbered
Disk 1, song 1
Disk 1, song 2
Disk1, ...
Disk 2, song 1
Disk 2, song 2
etc.
In this case, only the first songs on the first disk are shown...
Jaap