Nginx Show Upstream Server Name and Backend Port With URL Without Trailing Slash

I have setup a Nginx server as proxy for a back-end. If the back-end is down, Nginx serves from a backup of the back-end. The proxy works when the URL ends with a trailing slash. If I omit the trailing slash, the URL shown becomes the name of the upstream block plus the port of the backup back-end.

• What works: www.chingu.asia, www.chingu.asia/, and www.chingu.asia/wiki/.
• What doesn't work: www.chingu.asia/wiki (no trailing slash), my browser is redirected to http://chingu.servers:8111/wiki/, which cannot be found (of course).

Here is my configuration:

upstream chingu.servers {
    server 192.168.0.12      fail_timeout=300s  max_fails=1;
    server 192.168.0.10:8111 backup;
}
server {
    listen 80;
    server_name www.chingu.asia;
    location / {
        proxy_pass http://chingu.servers/;
    }
}
server {
    listen 192.168.0.10:8111;
    server_name _;
    root        /...some path.../Chingu;
    index       index.html index.php;

    location / {
        try_files $uri $uri/ =404;
    }

    location ~ \.php$ {
        try_files     $uri =404;
        include       fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_pass  unix:/run/php/php7.3-fpm.sock;
        fastcgi_param FQDN true;
    }
}

I have tried using regexes to match any URI with the location blocks. I also tried port_in_redirect off; and absolute_redirect off;.

I could add location block for wiki and others but it seems to defeat the purpose of a proxy if I must add manually every possible sub-directories.

What is it that I'm missing?

Your configuration means that nginx will use chingu.servers as the HTTP Host header when sending the request to upstream.

You need to use

location / {
    proxy_set_header Host $host;
    proxy_pass http://chingu.servers/;
}

in your configuration.

Also, you need to make sure that your Chingu root URL is set to www.chingu.asia.