Birthday Calendar - Add Second Alert

On the Birthday Calendar on my MacBook Pro (M3, 2023), I have birthdays set up to repeat annually with the default alerts "1 week before." Until a few weeks ago, I was able to add a second alert "On day of event (9 AM)." Now when I try to add a second alert, I get a message that says, "‭Calendar was unable to save changes to John Doe's birthday" with a message underneath that says "This calendar is read only." Birthdays that were previously modified with two alerts still have both alerts, but I can no longer modify birthdays with only one alert to add a second alert. Is there a workaround to add a second alert to birthdays on the Birthday Calendar?