Just seen that even after an addressbook have been deleted, birthdays of now deleted contacts of this addressbook have “survived” in “Contact’s birthdays” calendar.
As I’m testing with vcard import, I’ve deleted and recreated this addressbook , and imported the same vcards several times. Can see that birthdays are never deleted, but always added, n times imported, n entries.
IMHO a bug, but not sure of Contacts app, Calender app, or both?
Workaround is to disable creation of “Contact’s birthdays” calendar in admin settings, re-enable. This seems to delete the whole calendar, but the cron job recreates/repopulates it correctly