Got strange problem on logstash: configuration file reset automatically

Hi, I am a totally new beginner of ELK. At work I was asked to build a ELK monitoring system on company's servers. Here I got a very strange question. I have three configure files corresponding to input, filter and output in the folder /etc/logstash/conf.d/. The question is all these three configure files will be reset to its default after I restart the service or when I come to the company on next day after one night. But sometimes it does not occur. I am really confused about this. Do you know the possible reasons and have some suggestions?

Logstash doesn't modify its own configuration files. You'll have to look elsewhere for the cause.