What I found after a lot of digging around is that my nginx configuration was not taking into account the load balancer as a pool.

In my http block I needed to use upstream to identify the backend server addresses that are used for load balancing:

    upstream mauticWeb {
    least_conn;
    server ${MAUTIC_WEB_URL}:9000;
    }

and in my http --> server block wherever I needed to reach the backend php-fpm server I needed to use the upstream config.

server{
    ...
    location ~ ^(.+\.php)(.*)$ {
       ...
       fastcgi_keep_conn on;
       fastcgi_pass mauticWeb;
       ...
    }
    ...
}