Not sure if I have gone about this the correct way, but I am migrating my collection into MB from iTunes. I have a lot of autoplaylists set up in iTunes so have been recreating them in MB. To do this I have been exporting them from iTunes as m3u files and importing into MB so I have a static playlist, selecting all of the files in the playlist then editing my Custom7 field (Autoplaylist) and adding the relevant tag (i.e. Running, Rock, Mellow etc). Then when I create an Autoplaylist with the Custom7 tag as Running for example there are no files in it, however if I change the Autoplaylist source from Library to my static playlist the Autoplaylist works fine.
It would seem that when I am writing the Custom7 tag it isn't writing it to the full library, only a sub section.