Try add the following to site.conf.hbs (before the WordPress config segment):
location ~* /[wp\-]*sitemap.*\.xml {
try_files $uri $uri/ /index.php$is_args$args;
# return 200 "Match found \n";
allow all;
log_not_found off;
access_log off;
}
location = /robots.txt {
try_files $uri $uri/ /index.php$is_args$args;
allow all;
log_not_found off;
access_log off;
}
Source: