Zendframework - Adding an action to existing controller results in permission denied - php

Question

I'm using Zendframework (2.3), I'm struggling to understand what I'm doing wrong when attempting to create a new action and view on an existing controller. I've read some relevant documentation but still fail to see what I'm missing.
Currently the controller basically defaults to a single action (main), I would like to add an additional one.
EG:
/list-item/main => // Existing Route
/list-item/add => // New Route I would like to add.
This is how my module.config.php looks:
return array(
'router' => array(
'routes' => array(
'list-item' => array(
'type' => 'segment',
'options' => array(
'route' => '/list-item[/:action][/:id][/:id1][/:id2][/:id3][/:id4][/:id5]',
'constraints' => array(
'action' => '[a-zA-Z][a-zA-Z0-9_-]*'
),
'defaults' => array(
'controller' => 'ListItem',
'action' => 'main',
),
),
),
),
),
'controllers' => array(
'invokables' => array(
'ListItem' => 'ListItem\Controller\ListItemController',
),
),
'view_manager' => array(
'template_path_stack' => array(
__DIR__ . '/../view',
),
),
);
If I can read this configuration correctly, the actual action name is optional, but the module will default to the main action. Yet it is flexible to allow for any action to be attempted.
So, I proceeded to add a new public function into the ListItemController, assumed this is how the convention works:
public function addAction() {
return new ViewModel();
}
And with it a new view file into the module's views folder, in fact right next to main.phtml but called add.phtml.
But when I attempt to access the route /list-item/add I only get a permission denied. Funny, because the actual status is 200. But I have no other information. I honestly don't even know if this is a ZF thing, I can only assume.
Also, I'm using php -S 0.0.0.0:8080 -t public/ in case the web server might have something to do.
Thanks in advance, any help will be greatly appreciated.