Vaish disabled, but still add a custom header (hostname)

Why in Vaish 4.0 can't I see my custom header set on the response ?

sub vcl_recv {
   // disable vaish for now
   retu (pass);
}

sub vcl_backend_response {
    set beresp.http.X-Backend = "my_custom_hostname";
    retu (deliver);
}

It was working some times ago, (in vaish 3.x or without http2 on nginx backend, don't know which factor is it).

Do you know what's causing the fact that i can't see my X-Backend custom header in the http response ?

Thanks.

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