txtContent not showing in header's form

Hello !

I'm a new Sharepoint user so my issue must be something really idiot... But everything i tried failed.

I have done few lists and i'm currently formating the form for a list I created from an excel tab.

All of the auto generated columns have internal field name like : field_1, field_2 etc but in the list fields are the same as the excel file : Client, Title, etc.).

I used the doc here : Configure the list form | Microsoft Docs

When i customize the body form, no pb, field are working properly.

I paste the header content from the Microsoft Tutorial. I customized minors things like color, background etc.

But I can't find a way to have this original code working :  

"children": [
                {
                    "elmType": "div",
                    "txtContent": "='Contact details for ' + [$Title]"
                }

Here is mine : 

 "children": [
                {
                    "elmType": "div",
                    "txtContent": "='Client ' + [$Client]"
                }

I have no error but in the header the value is blank, empty. It's like it's returning nothing or can't find the field (The field is a single text field) The $Client should be visible next to the group Icon in the banner (Like Monika Bayak from the Microsoft Tutorial) :

On another form, the code works. The column Client (not the same) is a Lookupvalue. But even there, it's showing something strange. For the first line of the list the display is like 

1;#FirstName_LastName

With the [$Client], it's returnining at the beginning (I guess) the row ID.

I don't really understand what is going on here ! 😄

Thanks for your help  !