I found way to reproduce in production server. If we use a plugin which use a nested route of adminPlugins then we will not have the issue. In this case index route created automatically like @zogstrip mentioned . For example I am using discourse-navigation plugin which have page as /admin/plugins/navigation. So I am not having the issue in my live. Like it I guess meta.discourse will have akismet plugin.
If we don’t have any plugins which using that path then we can see the issue.