designerHost uses the wrong activity designer

I have the code shown below to assign a custom designer to my custom activity. The activity type shown is the type I use as the root activity type in my workflow.

When I later look at the rootDesigner using the following code

IRootDesigner rootDesigner = designerHost.GetDesigner(designerHost.RootComponent) as IRootDesigner;

then I can see that the RootComponent is of type SequentialWorkflowDesigner and NOT of my custom type WorkflowActivityDesigner. The constructor of WorkflowActivityDesigner is not being called.

Why does the designerHost not use my custom designer

or

How can I change the designer used for the root component

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[Designer(typeof(WorkflowActivityDesigner))]
public partial class WorkflowActivity : SequentialWorkflowActivity {

public WorkflowActivity()
{
InitializeComponent();
}

}

public class WorkflowActivityDesigner : SequentialWorkflowRootDesigner {

protected override void OnPaint(ActivityDesignerPaintEventArgs e){

e.Graphics.DrawString("test", new Font("Arial", 12), new SolidBrush(Color.Red), this.Bounds.Left, this.Bounds.Top );

}}