I was one of the ones who was unable to book FP+'s on a party day last year. We had tickets for the party only. We were not staying on site, if that is relevant. I have also read of people being able to book before the party with just party tickets. I kept getting a message in
MDE that said I couldn't book FP+'s even though my party tickets were linked to my account and magic band. I'm sorry that doesn't answer your question but FWIW that was my experience last year. This year I plan on being at MK the day of the party so it definitely won't be a problem for me. Sorry but this may be a wait and see situation.