I have the feeling that notification sytem dont look into extension langage folders for find template email.
Exemple, in UCP >> notification options, i enable email for my extention's notifications type.
In my vendor\ext\notification\type\exemple class, i have:
the notification is send, but not the email, with this error:
template() method in functions_messenger.php had a third argument for specify a template path, but it is not used in phpbb\notification\method/messenger_base.php :/