Product details action admin extension authentication

[Joe47]

My application has different urls for the frontend and the backend. Both urls are subdomains of the same domain. In a product details action admin extension I am trying to make an API call to my backend with fetch(). But the Authorization header with the JWT is not being added. It does get added if I make a request to my frontend server (which is my app url).

According to the documentation (https://shopify.dev/docs/api/admin-extensions#app-authentication) the Authorization header should be added should be added to any of my apps subdomains.

Here is an example of my setup:

Frontend subdomain: frontend.domain.com

Backend subdomain: backend.domain.com

In my Shopify app setup the "App URL" is https://frontend.domain.com/ and the "Allowed redirection URL(s)" is https://backend.domain.com/auth/callback

Am I missing something or should this be working?

Thanks