Not sure if this has been reported, but this past weekend I tried to do shuffle mode with some friends. We filtered down to songs with Keyboard and got 109 songs in the list. When we chose to shuffle these the setlist was only 68 songs.
It was automatically filtering songs based on Pro-Guitar even though we were using the Pro-Guitar as a regular guitar. It should let us play all the songs in the list.
Not sure if this matters, but it was on XBOX.