That's so weird?? Is she still in her house or walking around town? If not, it might have been time for her to go to sleep and maybe that's why the party is over.
But also if she is walking around town or in her house, this is a really weird glitch I didn't know about until now.
I don't think she'll be having her party while your gates are open. My husband has invited me over to his town before on one of his villager's birthdays and when I arrived, they weren't celebrating anymore either. I don't know if she'll go back to her party if you close your gates.