Controller naming PascalCase

If you develop on Windows, it finds files regardless of their case (camel, pascal, ransom).
That often breaks when running your app on Linux, as it appears you have discovered.
With URI routing, the deemed lowercase controller name has the first letter capitalized, and that is the source file that CI's loader looks for.
The proper way to get around that is with routing rules, which is the solution you came up with.

This is a feature of CI's routing.

Your mod_speling suggestion might help with Apache, but I don't know if the same is available for NGINX, nor do I suspect it would be easy to build into PHP's builtin server (thru our spark script).