I had my last day of FPs cancelled in Sept when I booked FPs before 6pm for MNSSHP on my day off (no regular park ticket that day). However, I followed the great advice given on this thread (or elsewhere on the Boards?) to check for FPs for that last day after using up those Party FPs. While I didn't check immediately (was enjoying MNSSHP), I would have been able to book FPs for my last day.
In my case, FPs were not that necessary or desirable because my last day was Epcot, it is a tiered FP system and I planned to ropedrop anyway. I did check a few times for FP availability for that last day and noted that I could have booked FPs if I wished to. Of course, the FPs a few days out for Epcot were not that great as Frozen, Soarin', TT were long since gone. That Epcot day was also the 35th Anniversary so extra demand and crowds.
The big takeaway here is that you can book FPs for your last day once you use your FPs on your party ticket. You will probably not have a great selection, but it is possible. If I had planned to be in MK that last day, I def would have made my FPs because of the number of attractions in the FP pool.
If big headliners like Frozen, 7DMT and esp Pandora's Flight of Passage are must do's, don't plan on riding those on the last day of your vacation if you are also planning to make FPs on your party ticket. If you have no choice but to book FPs for any of these Big 3 attractions on your last day, then don't book FPs on your party ticket. Harsh but looks like the present reality.
If anyone has had a different experience, I'd love to know about it. Would love to be wrong about this.