How to stop Monolog processing using a custom handler?

Question

I am using Monolog in a Symfony 2.8 project. I have configured a mail handler to send me notification about all errors.

Since not all errors are important (e.g. PageNotFound for /apple-app-site-association) I would like to add a filter. I did this by adding a custom Handler:

Config:

services:
    monolog.custom_handler.service:
        class: AppBundle\Log\CustomHandler

monolog:
    main:
        type: service
        id: monolog.custom_handler.service            
        level: error 
        handler: mail_buffer 
    mail_buffer:
        type:    buffer
        handler: mailer
    mailer:
        type:       swift_mailer
        from_email: "my address"
        to_email:   "my address"
        subject:    ERROR!
        formatter:  monolog.formatter.extended   

Handler:

namespace AppBundle\Log;

use Monolog\Handler\AbstractHandler;

class CustomHandler extends AbstractHandler {
    public function handle(array $record) {
        // Check if handling level was reached
        if ($record['level'] < $this->level) 
            return false;

        if (!empty($record)) {
            $url = (isset($record['extra']) && isset($record['extra']['url']) ? $record['extra']['url'] : '');
            $message = (isset($record['message']) ? $record['message'] : '');

            // Well Known Pages         
            if (strpos($url, '/.well-known') === 0) {
                //echo "...match 1</br>";
                return true;
            }

            if ($url == '/apple-app-site-association') {
                //echo "...match 2</br>";
                return true;
            }

            if (strpos($url, '/apple-touch-icon') === 0) {
                //echo "...match 3</br>";
                return true;
            }

            ...
        }

        // Record was NOT handled --> DO NOT stop bubbeling
        return false;
    }


}

Using the echo statements I can confirm, that the handler is called, and the if clause correctly matches. So, if /apple-app-site-association is called for example, the handler returns true. I would expect, that this stops bubbeling, and that thus the nested mail_buffer handler is NOT called anymore. This is not the case.

No matter if the handler return true or false, I still receive mail for all error.

What am I doing wrong? How can I stop the processing of filtered messages?