Please refer to the attached gif file.

Workaround:
 this.radGridView1.GridBehavior = new CustomBaseGridBehavior();


public class CustomBaseGridBehavior : BaseGridBehavior
{
    public override bool OnMouseMove(MouseEventArgs e)
    {
        GroupPanelSizeGripElement grip = this.GridViewElement.ElementTree.GetElementAtPoint(e.Location) as GroupPanelSizeGripElement;
        if (grip != null)
        {
            this.GridViewElement.ElementTree.Control.Cursor = Cursors.SizeNS;
            return true;
        }
        
        return base.OnMouseMove(e);
    }
}