Could be a bug with W3TC. To overcome this, you may use those URLs hardcoded in nginx configuration like this...
server {
server_name domainname.com;
# other directives
location = /robots.txt {
try_files $uri /index.php;
}
location = /sitemap.xml {
try_files $uri /index.php;
}
location = /sitemap.xml.gz {
try_files $uri /index.php;
}
}I hope this helps.