Elastic Beanstalk Nginx not receiving POST requests

I have 3 Elastic Beanstalk, load balanced instances running a NodeJS application behind an Nginx proxy. A while ago, I came across a very strange error. One of the instances was not receiving a post request. Here is what was happening.

My application login page was loading correctly through a GET request. Once I input the credentials and hit the login button, I could see the POST request being made to the server. Nothing would happen and the request would time out after a while. I ssh-ed into that instance and checked the Nginx access logs. It seems the GET requests were going through but the POST request was not even hitting the Nginx server. The same code was deployed on other 2 instances and was working fine.

I restarted that instance and everything was back to normal.

My question is what could be causing such a strange behavior where only the POST requests were not going through ?