To reproduce: add a RadForm and start the application.

Workaround:
public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();
        
    }

    protected override void OnLoad(Size desiredSize)
    {
        this.ElementTree.InitializeRootElement();
        this.RootElement.ApplyShapeToControl = false;
        base.OnLoad(desiredSize);
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        this.RootElement.ApplyShapeToControl = true;
    }
}