Steps to reproduce the issue

Install a libraries plugin or media plugin.

example: Kunena 6.0 Libraries

on kunena.xml file has

<namespace>Kunena\Forum\Libraries</namespace>

Expected result

autoload_psr4.php has the libraries namespace.

Actual result

No libraries or media dir has been added to the autoload_prs4.php file.

other namespaces has been added:

'Kunena\\Forum\\Site\\' => [JPATH_SITE . '/components/com_kunena/'],
'Kunena\\Forum\\Api\\' => [JPATH_API . '/components/com_kunena/'],
'Kunena\\Forum\\Administrator\\' => [JPATH_ADMINISTRATOR . '/components/com_kunena/'],
'Kunena\\Forum\\Plugin\\Kunena\\Altauserpoints\\' => [JPATH_PLUGINS . '/kunena/altauserpoints/'],
'Kunena\\Forum\\Plugin\\Kunena\\Community\\' => [JPATH_PLUGINS . '/kunena/community/'],
'Kunena\\Forum\\Plugin\\Kunena\\Comprofiler\\' => [JPATH_PLUGINS . '/kunena/comprofiler/'],
'Kunena\\Forum\\Plugin\\Kunena\\Finder\\' => [JPATH_PLUGINS . '/kunena/finder/'],
'Kunena\\Forum\\Plugin\\Kunena\\Gravatar\\' => [JPATH_PLUGINS . '/kunena/gravatar/'],
'Kunena\\Forum\\Plugin\\Kunena\\Joomla\\' => [JPATH_PLUGINS . '/kunena/joomla/'],
'Kunena\\Forum\\Plugin\\Kunena\\Kunena\\' => [JPATH_PLUGINS . '/kunena/kunena/'],
'Kunena\\Forum\\Plugin\\Quickicon\\Kunena\\' => [JPATH_PLUGINS . '/quickicon/kunena/'],	
'Kunena\\Forum\\Plugin\\System\\Kunena\\' => [JPATH_PLUGINS . '/system/kunena/'],

Also i tried on system plugin:

class plgSystemKunena extends CMSPlugin
{
	/**
	 * Method to register custom library.
	 *
	 * return  void
	 */
	public function onAfterInitialise()
	{
		\JLoader::registerNamespace('Kunena\\Forum\\Libraries', JPATH_LIBRARIES . '/kunena/');
	}
}

System information (as much as possible)

4.0-dev git

Additional comments