For my albums that are multi-disc, some files use the mp3 tag attribute disc to indicate which disc the song is from.

I use an active playlist for album artist w/ the following keyword hierarchy. (sorted by %SORTTITLE%)
Album Artist
  |-Album / Series

I noticed than when are tagged disc 1 vs disc 2, dlna clients see them ordered by their song #.

like so:

disc 1 song 1
disc 2 song 1
disc 1 song 2
disc 2 song 2
.. and so on

I can add disc/season to the hierarchy, songs that are tagged with the disc tag now up in the mezzmo library tree and dlna clients as such:

album artist
     |-disc 1
     |-disc 2

But now albums with no disc metatag are simply filtered from the active playlist and do not show up.
I tried to add a smart filter to allow for blank disc values ("is <blank>) but it threw an error.

I expected files with no disc tag to simply show up under the album name without any disc identifier in the tree. Id rather not tag all my music w/ the disc number, or name the album tag disc 1 / disc 2 respectively, if I can avoid it.

A similar issue was reported a while back in this thread:

Does anyone have any thoughts?