As of nginx version 1.27.3 non-enterprise versions can also use resolve parameter on server inside upstream block to make it re-resolve.

upstream http_backend {
    server custom.domain.tld:443 resolve;

    keepalive 16;
}

server {
    ...

    location /http/ {
        proxy_pass https://http_backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        ...
    }
}