I am on 14.04, Nginx was not accessed for weeks and all of a sudden the default index.html replaced the one we had. We checked logs and no one accessed Nginx except us just now to change it back. No updates were made as far as we are aware.
Another question was asked with the same problem (Nginx index.html gets overwritten with default index.html file) and it was suggested I ask a new question for this same problem.
The only "answer" I could find was from https://www.digitalocean.com/community/questions/my-index-html-file-in-nginx-automatically-gets-overwritten-with-the-nginx-defaul-index-html-file-for-no-reason that suggest changing the location of the html files from /usr/share/nginx to something else (I like /var/www/nginx) and then making the modification in the nginx/sites-enabled/conf file.
We made this modification in the server but we can only wait for it to happen again to know if it is the solution. If anyone can help figure it out it would be greatly appreciated.