An addendum...

It appears that on -some- of the improperly sorted playlists, they're sorted in the wrong order perhaps because the songs have both 'part' and 'tracknumber' tags (Vorbis files). They're classical songs, and I've used 'tracknumber' as the track out of the CD, and 'part' as the part of a given work (eg a Beethoven sonata). So, some of the playlists seem to be sorted first by part, then by tracknumber. So, all the part 1's come first, then all the part 2's, etc... so the ultimate order is wrong.

What I don't understand, of course, is why it isn't simply sorted based on the order of the songs in the original m3u file... I thought that's what source_list.seq_no was for!