Laravel Queue Notifications

I have 2 environments, QA (1 webserver) and Prod (2 webservers behind a load balancer) that have been set up the same way using a database queue.

On QA everything works just fine.

On Production, I have some strange behaviour...Mailable works fine using the Queue but an email Notification doesn't work with the queue.

If I remove the queue from the notification, the email gets sent.

On QA, I can see both jobs being created in the jobs table. On Prod, only Mailable gets created in the jobs table.

Example with Activation Email:

<?php

namespace App\Notifications;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notification;

class SendActivationEmail extends Notification implements ShouldQueue
{
    use Queueable;

    protected $token;

    /**
     * Create a new notification instance.
     *
     * SendActivationEmail constructor.
     * @param $token
     */
    public function __construct($token)
    {
        $this->token = $token;
        $this->onQueue('social');
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return ['mail'];
    }

    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->subject('Activation email')
            ->greeting('xxxxxx - Hello!')
            ->line('You need to activate your email before you can start using all of our services.')
            ->action('Activate Email', route('authenticated.activate', ['token' => $this->token]))
            ->line('Thank you for using our application!');
    }

    /**
     * Get the array representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function toArray($notifiable)
    {
        return [
            //
        ];
    }
}

My .env file:

QUEUE_DRIVER=database

The file config/queue.php:

<?php

return [

    /*
    |--------------------------------------------------------------------------
    | Default Queue Driver
    |--------------------------------------------------------------------------
    |
    | The Laravel queue API supports a variety of back-ends via an unified
    | API, giving you convenient access to each back-end using the same
    | syntax for each one. Here you may set the default queue driver.
    |
    | Supported: "null", "sync", "database", "beanstalkd",
    |            "sqs", "iron", "redis"
    |
    */

    'default' => env('QUEUE_DRIVER', 'database'),

    /*
    |--------------------------------------------------------------------------
    | Queue Connections
    |--------------------------------------------------------------------------
    |
    | Here you may configure the connection information for each server that
    | is used by your application. A default configuration has been added
    | for each back-end shipped with Laravel. You are free to add more.
    |
    */

    'connections' => [

        'database' => [
            'driver' => 'database',
            'table' => 'jobs',
            'queue' => 'default',
            'expire' => 60,
        ],

And my Mailable that works with queue:

Mail::to($user->email)
                        ->queue(new Welcome($user));

Many thanks for your help

via Chebli Mohamed