Hi Sander,
no, before the task activity, you are assigning the task to someone, see my example screenshot - there is an "assign to" activity before the task.
This "assign to" activity is currently set to either a user or a group, that is why your substitution rule does not work.
You need to change the "assign to" activity to assign to the substitution rule instead.
The change you did in your screenshot "settings for abscence.png" only reassigns the task after the task due date is reached and does not interact with the substitution.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant