Just to add to the fray here, which I may regret judging by the time of some posts.....
However, I have seen numerous reports (on both touring plans and dis) of people losing their last day of fastpasses, after having used their party tickets to make fps on a party day. It seems the system updates, made in June this year, have meant that the party tickets are being ignored in the overnight "sweep up" process, thus matching the fps to a day of theme park tickets instead.
As an example: imagine a 4 day ticket and party ticket making 5 days in all. Party day on day 3. They make fps on day 3, as people have been able to do before, and days 1, 2, 4 and 5. Overnight checks result in a "you have too many days worth of fps for ticket media" message, followed by deletion of day 5 fastpasses. CMs giving all sorts of contradictory advice. Officially though party tickets do not include the use of fastpasses, as there are none during the party. It just so happens that it has been well known that you could use them for fps before the party starts.
This is not happening universally, so no way to know why. It is reasonable to assume it is to do with the system updates in June to close the loopholes, but whether deliberately stopping party tickets being used for fps, or an unintentional consequence is unclear.
Nevertheless it is happening, to some people. Please don't shoot the messengers! I saw this first on TP, and had a look on DIS to see if this was widespread.