Steps to reproduce the issue

JLoader::registerNamespace('B', JPATH_SITE . '/libraries/app/vendor/B/src', $reset = true, $prepend = false, $type = 'psr4');
\B\B::anyMethod();

Expected result

Autoloading \B\B class and calling anyMethod.

Actual result

Class not found.

Additional comments

File: libraries/loader.php

1. if (strpos($class, $ns) === 0) is not enough, there can be two namespaces AAA and AAAB, both starting with 'AAA'. Valid check is:

$nsLen = strlen($ns);

if (strpos($class, $ns) === 0 && isset($class[$nsLen]) && $class[$nsLen] === DIRECTORY_SEPARATOR)

2. $classFilePath = $path . DIRECTORY_SEPARATOR . str_replace($nsPath, '', $classPath);
   It's totally wrong to replace namespace name in fully qualified class name. Consider \Apple\AppleBridge\AppleNice\AppleClass

Valid behavior is to strip only leading namespace part:

$classFilePath = $path . DIRECTORY_SEPARATOR . substr($classPath, $nsLen + 1);