Why a Workflow Owner cannot See Update Task Due Date Option in an Object workflow task in Vault?
The Update active task due date when date field is updated option is checked in the Object workflow task configuration.
When selecting this option, Vault does not allow manual updates to the due date.
Example: If the due date is controlled by the Release Date field, selecting this checkbox updates the due date for this task every time the Release Date field updates. When a task due date change occurs this way, Vault audits the change as System and updates the Last Modified field on the task.
Vault Help Documentation: Calculated Task Due Dates