When the same problem also happens in the Web browser and with Apple, it’s probably Nextcloud related. Did you already ask there?
How can I enable a reminder for my contacts birthdays? I can see the birthdays of my contacts in my google calendar app, but I cannot set a reminder on them. I tried to set “default reminders” in the calendar settings but I guess this is only the default for newly created entries of a calendar.
It’s embarrassing to forget to congratulate people you like, just because you do not have their birthdays in mind…