macOS 26.5: Birthdays from contacts are no longer showing up in the birthday calendar

macOS 26.5: Birthdays from contacts are no longer showing up in the birthday calendar. If I open an individual contact and temporarily change the birth date, it suddenly reappears in the calendar. I have already toggled the corresponding checkbox on and off multiple times and also restarted the system. None of that helped. What can I do? I can’t realistically modify every contact temporarily...