this is by design. Right click -> reassign only reassigns this one instance of the task while the instances of the other users are not changed.
If you want to reassign a task from a group to a single user, you should do that by creating a decision "reassign" in the task which asks for the user to send the task to. Then, on the "assign" tab, assign the next task to the entered user. At last, connect this decision back to the task itself.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant