I have only used this once in a plugin, in Harmonics Playback, and that turned it off. I could not really tell you if it works. Assuming you are calling it with the specified parameters and it does not work, it may well have never worked.
I will try a quick experiment when I have a chance. Supposedly someone form Avid monitors the Sibelius user group on Facebook so it would be good to report it there.
I have had very little success in trying to get plugin bugs fixed though.