New event -> date of event from the past (e.g., March 1, 2020). Repeatability -> Monthly -> Every 1 month -> On every 3rd day of the month.
The event was recorded for every 1st day of the month, not on the 3rd day.
This error does not occur if the created event is for the future.
In general, there seems to be a lack of logic regarding repeatability for the days of the month. Maybe here: https://github.com/kolab-roundcube-plugins-mirror/libcalendaring/blob/master/lib/Horde_Date_Recurrence.php#L1213