Permission Changed!

Hi, i have problem with Config.php file permission
I changed file per to 644 and after 24h or 30h permission changed to 400 automatically! And receive this following errors:

Warning: include(/.../public_html/config.php): failed to open stream: Permission denied in /.../public_html/index.php on line 2

Warning: include(): Failed opening 'config.php' for inclusion (include_path='.:/usr/local/php56/lib/php') in /.../public_html/index.php on line 2

Notice: Undefined variable: sitepath in /.../public_html/index.php on line 4

Notice: Undefined variable: secret in /.../public_html/index.php on line 5

Notice: Undefined variable: path in /.../public_html/index.php on line 34

Notice: Undefined variable: path in /.../public_html/index.php on line 35
Invalid Path

Help me plz

Did you check Apache access_logs ( /var/log/httpd/access_log) or /var/log/httpd/domains/domain.com.log to see IP address from where permissions are getting changed?