Server receives POST request twice from Nginx - nginx

Question

We have a nginx server acting as a reverse proxy between the client and server.
Whenever the server returns a 500 we actually see that the request is being sent to the server twice from the nginx logs:
173.38.209.10 - - [26/Jan/2018:15:15:36 +0000] "POST /api/customer/add HTTP/1.1" 500 115 "http://apiwebsite.com" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36"
173.38.209.10 - - [26/Jan/2018:15:15:36 +0000] "POST /api/customer/add HTTP/1.1" 500 157 "http://apiwebsite.com" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36"
This API is only called twice if the first response is a 500.
If I bypass the nginx proxy and call the server directly, then it's only called once.
What's more strange is after further testing we found out this only happens in our corporate network. If i use my home network to connect to the proxy, then there's no retry even in case of a 500 response.
Anway, here's my nginx configuration:
server {
listen 80 default_server;
listen [::]:80 default_server ipv6only=on;
root /usr/share/nginx/html;
index index.html index.htm;
# Make site accessible from http://localhost/
server_name localhost;
location / {
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header Host $http_host;
proxy_pass "http://127.0.0.1:3000";
# Uncomment to enable naxsi on this location
# include /etc/nginx/naxsi.rules
}
location /api/customer/ {
proxy_pass "http://127.0.0.1:8080/";
}
}
Is there anything suspicious which is causing this behaviour?
Thanks