I started the RB1 Throwback Challenge earlier this week and noticed something "odd." I've been playing the challenge solo and noticed after playing every song in alpha order that 3 didn't register. When I finally figured out which songs they were, I noticed they were songs that I'd played before starting the challenge and had registered a score. On the second (plus) playthrough, my score was lower than my previous high score and therefore didn't register as being played.
So the question is, designed that way or bug? Seems buggy to me since all the requirement is to play the song, not beat a previous score.