The Due Date field changes are not reflected on System Notes and it is a system limitation.
It is logged under Enhancement 189337 Changes made in Due Date is not being tracked in the System Notes.
As an alternative solution:
- Create a new Transaction Body Field called ‘Copy Due Date’. Type of the field is Date.
- Create User Event script on After Submit which copies value from Due Date to Custom Field.
- In System notes will see changes in Due Date under the custom field ‘Copy Due Date’