How to prevent nginx from replacing my index.html with default nginx welcome page.

If you are running Ansible you want to make sure that your server config management isn’t overwriting that file itself. You can also check what user the directory and file are owned by. It maybe a permissions issue, it maybe config management overwriting something since you mentioned Ansible, it maybe you are running that directory as nobody:nogroup with 777 permissions and perhaps there is an exploit somewhere.

But this should not be occurring - files don’t get overwritten - and DigitalOcean droplets aren’t fully managed - which means after they are deployed they are yours to use and there is no file level access that DigitalOcean retains to the droplet after it is running.