COVID shutdowns have made this an issue for A LOT of events, including mine.
Looking at the essential purpose of calling it an "annual" event, it's because you're defining it as an event offered once a year on a regular basis. Everyone had to skip last year and maybe this year, but it's still on an annual schedule if you're normally doing it once a year. You're not saying how many times you've had it, that would simply be the number with out "annual."
In my case, it would have been the 11th year, but I still called it the "10th Annual" event because I'm saying "This event that is offered once every year, which people have liked enough for the last 10 times that you can plan on it being offered again this time next year."