When the catalog task is created through flow designer and even though we are setting up the duration the due date is showing up as empty.
What you experience is an OOB behavior.
Due date is not automatically set on catalog task when a Flow is used, as it would be set with Wofkflow Editor.
In Workflow editor, when no due date is specified by user, it is calculated from the '_setDueDate' function in "WFCreateTaskActivityUtils" script include as the created time of task.
But in flow designer, unless the due date field is specified by a user, a default due date is not set. This is done intentionally by design.
You would need to implement your own logic to populate the due date into the catalog task see screenshot.
This would be a good candidate for the Idea Portal. Our product team will evaluate the idea further and look into ways this can be updated in our upcoming releases. You can submit it here - https://community.servicenow.com/community?id=community_static&content_id=91acf933db9ff740d82ffb24399619f5