When you have a reverse proxy in front of Tomcat that denies access to the /manager endpoint, then it might still be able to access that page by exploiting path normalization inconsistencies between tomcat and the reverse proxy.
Tomcat will treat the sequence /..;/ as /../ . However, reverse proxies such as Nginx will not normalize this sequence and send it to Tomcat as-is. This allows you to access paths that are otherwise denied by the reverse proxy.