svn auth caching mecanism relies on nginx fastcgi_cache. However, as noted in fastcgi_cache_valid documentation, when the upstream (/svn-project-auth internal route) sets a Set-Cookie header, there is no cache possible.
When dynamic credentials is used, the plugin hooks on beforeLogin and start a session if it's not already open.
When svn auth route is used with dynamic credentials plugin, there is a session started, hence no cache possible