Template start expression empty result

[d1scover]

Hi ! 

I'm new with App Sheet, and I'm struggling to use the <<Start: >> expression in my Google docs template.

I have 2 tables, "Invoices" : 

and "Services" :

There is a one-to-many relationship between "Invoices" and "Services" : an Invoice contains different services, and a service is only linked to one invoice.

My goal : for every created invoice, generate a PDF that lists the invoice's information, and the related services.

To list the related services, I understand I have to use the <<Start: > expression.

If I understand correctly, I try to achieve "Reverse reference" as described in the doc : https://support.google.com/appsheet/answer/11541779

So here's what I tried in my google docs template file : 

<<Start: [Related Services]>>

<<[Service Name]>>

<<End>>

But this produces an empty text on my generated PDF.

Anyone knows what I'm doing wrong ?

Thanks for your help !