Set the substitute on the role-document and it will work.
In your case you are routing to a role and not to a person,
in that case a substitute needs to be specified on the role.
This is (little) explained in the Redbook "Using Domino Workflow" from 2000. http://www.redbooks.ibm.com/abstracts/sg245963.html?Open
see page 180 of the PDF