My playback of diddles on any type of tuplet plays back as flams. On any regular notes (eighths, sixteenths) it is fine and exactly doubles the rhythm of the note. Anyone know what's up?

Sibelius 2 has a bug with diddle playback on tuplets, which is most likely what you're hearing. It creates an excess of attacks on any partial other than the first partial of a triplet (or other tuplet) when a slash is applied. Fortunately, Sibelius 3 has fixed that problem.