Birthdays from cardbook appear twice in lightning with 31 days offset.
I use the “birthdays” function (last item in settings) to synchronize the birthdays (and anniversaries and days of death) with the calendar (settings below for reference). This works, the birthdays show up in the Lightning calendar. However, in addition to the correct day, the appear again 31 days later. This does not happen for all birthdays but only the ones that are synced in the window at startup.
So a birthday on 19900101 will correctly appear on January 1st, 2021, but then again on February 1st, 2021. A birthday on February 2nd appears again on March 5th (+31 days). The vcard has no duplicated entry but the calendar has.
all address books
birthday, anniversary, day of death
days for searching: 30
show pop-up at startup, not daily and show if empty