Tuning property is rounded off to the nearest semitone

Listen to the attached score. Note that I've increased the "tuning" property of the notes by 10 cents each measure. However, there's no difference in playback until it hits 50 cents, at which point playback abruptly jumps up a full semitone. I wasn't sure if this was an issue or intentional decision with a piano (typically incapable of microtonality, so I replicated the same thing with the solo violin, an instrument that is capable of continuous tuning in real life. Same result--tuning rounded off to a semitone.