Why does WPF Style to show validation errors in ToolTip work for a TextBox but fails for a ComboBox?

Question

I am using a typical Style to display validation errors as a tooltip from IErrorDataInfo for a textbox as shown below and it works fine.

                      
        
        

          

            
            

              
              

            
    


                                 
              
            
                          
    

        
         

                验证码
                

                  
                
                

                   看不清?
                
              
                                  

                    
   
                 
             
              
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