The plugin Get Page Count is usually not needed, since the \$NumPages\ wildcard gets the same data and automatically updates when the number of pages changes. The plugin is only useful when you need to make adjustments to the number of pages in the score.
This update adds a fixed offset adjustment to the number of pages, after discovering that in some cases the plugin gets the wrong answer when determining the number of title and other blank pages before the first bar in the score.
If the full score and every part has the same number of leading blank pages, you can use the offset instead of the title page adjustment. In that case use a negative adjustment.
Since the plugin processes the full score and all parts, this will not help if the score and parts have different numbers of leading blank pages.
The traced outputs now give more information about the number of pages the plugin is detecting.
My current impression is that some bug in Sibelius is confusing the number of blank pages.