Disney's new FP system is cracking down on any FPs made that are not made with a valid PARK ticket used for entry that day.
If you don't have a park ticket for the day of the FPs you've chosen the FPs will be invalid, and it seems in this case deleted by the system. Whether or not you have a party ticket (Halloween or Xmas) won't matter. There are no FPs during the party hours, and whether or not you may, as a courtesy, enter MK earlier than 7pm, you are not entitled to a FP if your entry is based on a party ticket only. The party officially starts at 7pm.
If you DO have an MK ticket for park entry that day, as well as a party ticket, the fact that
MDE won't allow you to book 3 FPs with your MK ticket that day, and another 3 FPs with your party ticket for that day, tells you you're not entitled to FPs with the party ticket.
Many tried circumventing this by not linking their party tickets to their same MDE account used for their park tickets, but instead creating a separate MDE for just the party tickets. Disney is cracking down on people trying to get more FPs than they are entitled to have (regardless of what was allowed by the system in the past).
Personally, I think it's fair that you can't book FPs in MK on party days if you don't use a PARK ticket for entry. Early entry party guests can still go Standby, while others that paid for park entry that day can book FPs until park closing, when they are kicked out.
I do think Disney may try to be accommodating to people as this is all worked out, but probably only for a little while.