Bizarre case sensitivity cropped up in url interpretation

I've had a controller called Test in my application/controllers directory for some time. A recent change has suddently caused a problem where I get 404 not found when I request this url:

Code:

https://example.com/test

But if I capitalize Test then it loads just fine

Code:

https://example.com/Test

WHAT THE HELL?

This has been working fine for some time. The controller's file name is Test.php, the class name is Test -- but this is no different than all my other controllers that show no case sensitivity. What the heck could have changed? And why does it affect only this one class?