Autoloading in 5.7.4
I get the impression the autoloader looks only for certain folders in your package (/controllers, etc) and doesn't try to resolve the location by parsing the full class name. It does look like 5.7.4. is looking in a 'Src'-folder in your package (this wasn't the case in 5.7.3.). So classes in this folder do get autoloaded but not if they have underscores. So a class 'Test' in /my_package/src/test.php will be autoloaded but class 'TestTest' in /my_package/src/test_test.php won't.
I can't find any documentation concerning the issue.
Here are the requirements for PSR-4 which pretty much explains your class naming issue.
Sweet. Thanks for that link.