In lsw_absence.trg_absence_insert trigger, the {lsw_absence}.[absence_date] is populated with actual dates of the absence (skipping weekends, public holidays, etc.). This information is used to display the number of taken days, e.g. in the list view of the lsw_absence.absence_all_list form.
The referred trigger is called only for the insert event. However, the user_admin role can update the existing absence row and change the dates (lsw_absence.absence.date_from and lsw_absence.absence.date_to). Such an update operation in lsw_absence.[absence] doesn't source update in the {lsw_absence}.[absence_date].