"Assign Resource" pushbutton behaviour?

Hello!

We are trying to understand the behaviour of pushbutton 'Assign Resource' that you can find in both views Structure->Role (on a task) or Resources->Tasks.

Once a project role has been staffed with one or several resources by the staffing manager, the status is set to "staffing complete". We would like then to assign one of these resources to a dedicated task linked to the role, but the button is disabled.

Why don't we have the possibility to assign a resource to a task after Staffing Completion? Is there an other/better way to achieve this?

Thank you very much.

Matthias