Hello,
According to https://experienceleague.adobe.com/en/docs/workfront/using/manage-work/tasks/task-duration-and-durat... , using the duration type Simple determines, that the Planned Hours and the Duration are the same and base on the number of hours each assignee is allocated for.
My first idea was to add an Update Record module in the next step to manually define those values. But unfortunately this does not seem to work either.
So I would check your workflow and requirements, if the chosen duration type Simple is really necessary here.
Regards
Lars