WPF Validation ErrorTemplate for Custom TextBox

Question

Branching off of this question -

When attaching a validation error template to my custom textbox like this -

Answer 1

Alright so the way I managed to fix the issue was by removing the AdornedElement keyword and changing the error template as follows:

    
        
            
                
                    
                
                !
            
        
    
    
        
    

What I don't understand is why it behaves differently when using the AdornedElement keyword but works fine when binding the Tag/Tooltip using the RelativeSource. While the problem is solved, I would welcome any ideas as to why this happened.

Thanks