NGINX gets stuck on a specific location

Question

We have an nginx server that we're using as a reverse proxy. We redirect every HTTP traffic to HTTPS, except a specific path - /success.html. The mechanism works at first, but after a while (few hours / days), the /success.html location stops working and times out. The other location (/) keeps working well. After reloading the nginx, things return to normal (until next time...).

Our conf file:

server {
        listen                         80;
        server_name                    mypublicdomain.com;
        location /success.html {
                proxy_pass http://myinternaldomain.net/success.html;
        }
        location / {
                return 301 https://mypublicdomain.com$request_uri;
        }
}
server {
        listen                         443 ssl http2;
        server_name                    mypublicdomain.com;
        ...
        ...
}

What are we doing wrong here? We've seen similar issues when putting more than one location on the same server (with one location being more specific that the other).