Steps to reproduce the issue

I have a package with admin/site/library/plugins/media installers.

I want to use a namespace on the pkg script on the postflight.

use Kunena\Forum\Libraries\Install\KunenaModelInstall;

On the postflight:

$installer = new KunenaModelInstall;
$installer->createMenu();

Expected result

That the namespace is already registered at autoload_psr4 file.

Actual result

Class 'Kunena\Forum\Libraries\Install\KunenaModelInstall' not found

System information (as much as possible)

Nightly build J4

Kunena 6.0 nightly build: www.kunena.org/download on the bottom is the nightly build.

Additional comments