I have "Allow unassigned issues" set to OFF. When I go to the project roles page and try to set the default assignee, it correctly doesn't let me set it to Unassigned. However when I go to add a component, I'm able to set the default assignee for the component to "Unassigned". Then when I try to create issues with that component leading the assignee set to "Automatic", they correctly fail.
You should not be allowed to select "Unassigned" as the default assignee for a component when unassigned issues are not allowed.