Ajouter en-tête toujours dans le proxy inverse nginx échoue dans certaines situations

Dans la configuration suivante, lorsque le serveur en amont renvoie une réponse autre que HTTP 200 OK, la requête vers hostnameb.example.org ne contient pas l'en-tête X-custom-server-info.

La réponse de hostnamea.example.org est correcte (y compris l'en-tête).

Une idée de pourquoi ou comment le corriger?

worker_processes auto;
worker_rlimit_nofile 8192;
pid /opt/bitnami/nginx/tmp/nginx.pid;
events {
    worker_connections 4096;
}
http {
        include /opt/bitnami/nginx/conf/mime.types;

        add_header X-custom-server-info "someinfo" always;

        proxy_set_header         X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header         Forwarded "";
        proxy_http_version       1.1;
        client_max_body_size     10m;
        client_body_buffer_size  128k;
        proxy_connect_timeout    180;
        proxy_send_timeout       180;
        proxy_read_timeout       180;
        proxy_buffers            32 4k;

        proxy_set_header         Host                $host;
        proxy_set_header         X-Forwarded-Host    $host;
        #proxy_set_header         X-Forwarded-Proto   $scheme;
        #proxy_set_header         X-Forwarded-Port    $server_port;
        proxy_set_header         X-Forwarded-Proto   https;
        proxy_set_header         X-Forwarded-Port    443;

        server_tokens off;
        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        server_names_hash_bucket_size 128; # ceci semble être nécessaire pour certains vhosts

        default_type application/octet-stream;

        resolver dns-default.openshift-dns.svc.cluster.local valid=300s;

        server {
          listen 8081;
          server_name  hostnamea.example.org;
          set $bff "http://bff.project.svc.cluster.local";

          location / {
            proxy_pass               $bff;
          }
        }

        server {
          listen 8081;
          server_name  hostnameb.example.org;
          set $bff "http://bff.project.svc.cluster.local";

          location / {
            proxy_pass               $bff;
            add_header         Access-Control-Allow-Origin                https://external.example.org always;
            add_header         Access-Control-Allow-Credentials           true always;
          }
        }

        log_format upstreamlog '[$time_local]  "$request" UA:"$http_user_agent"';
        access_log /opt/bitnami/nginx/logs/access.log upstreamlog;
        error_log /opt/bitnami/nginx/logs/error.log;
}