I've now seen this three times, and have reproduced it at least once.
I go into the player and select to play music by Genre. I choose one of my genres, such as soundtracks. This brings up a list of something like 100 songs or so. I choose to play all the songs.
The resulting playlist, however, ends up being a random size. Sometimes it's twenty, sometimes 10. I even got one that was just 3 songs long.
I went into the menu again and tried selecting one of the songs that gets omitted, and it plays fine.
MKIIa 10GB - 2.0b11 w/Hijack
MKIIa 60GB - 2.0 final w/Hijack