Why doesn't the Absence Code get updated when using Mass Add Attendance, either from the Attendance Management page, Mass Change Period Absences page, or from Mass Change Attendance codes page?

The functions to update the Absence Codes on those pages will only update a blank, unless the option of Populate All Day Code Automatically has been enabled on School Settings. If that option is enabled, then in addition to blanks, the unverified absence (usually code A), will also update. 

Please note, these forms do not update other absence codes.