Fauxton behind Nginx is not fully working, what are the expected headers?

Question

Fauxton (CouchDB manager) behind Nginx is not fully working. Following Nginx block from CouchDB Wiki is not working at all:

location /couchdb {
    rewrite /couchdb/(.*) /$1 break;
    proxy_pass http://localhost:5984;
    proxy_redirect off;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}

When I change the location /couchdb part with location ^~ /couchdb, it starts working by displaying Fauxton at least.

Inspection (Chromium Inspector) shows the problem as the /_session url is requested from https://example.com/_session instead of https://example.com/couchdb/_session, which means that Fauxton can not determine correctly that it is served within a subdirectory (adding a proxy block for /_session url solves the 404 issue) That's why I dumped the request on CouchDB host with nc -l -p 5984 to see what the headers are set to:

GET /_utils/ HTTP/1.1
Host: example.com
X-Forwarded-For: 127.0.0.1
X-Real-IP: 127.0.0.1

I'm not sure if X-Forwarded-For header is set correctly or not. What is the expected headers here? How can I determine if the problem is with my Nginx setup or there is another issue with - for example - my firewall (iptables) settings?

Edit

Possible problem is with Fauxton, not with CouchDB: https://github.com/apache/couchdb-fauxton/issues/944