RadSplitButton does not get a focus rectangle when focus by the Tab key. One should first click the action part and then the focus rectangle will appear..

The current workaround is:
public class MyRadSplitButton : RadSplitButton 
{ 
    protected override void OnEnter(EventArgs e) 
    { 
        base.OnEnter(e); 
  
        this.DropDownButtonElement.ActionButton.Focus(); 
    } 
}