Workaround: Use a custom editor and override IsModified method: 
class MyEditor : RadDropDownListEditor 
{
 public override bool IsModified 
{
 get 
{
 return true; 
}
 }
 }

 void radGridViewFilter_EditorRequired(object sender, EditorRequiredEventArgs e) 
{
 if (e.EditorType == typeof(RadDropDownListEditor)) { e.Editor = new MyEditor(); 
}
}