The attendance manager can inadvertently make this change in in onRecord > Settings > Attendance > Attendance Types > Absent.
Tip: Since the default Absent - Excused has no sort order number, it always appears as the first Absent attendance type.

Example of the default Absent Unexcused attendance type inadvertently changed to an Excused attendance type.

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Example of how the default Absent attendance type needs to look:

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Note: As an attendance manager, you have access to change the title of the Absent reason and Excused/Unexcused. However, it is best to keep the Excused/Unexcused set to Unexcused.