magento2.4 – Why does RMQ Topic handler is never triggered?

I’m getting stuck with my Topic Handler in Magento.

I have a working RMQ consumer, I can read message on the Queue, no problem.

Here is the communication.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <topic name="shipmentQueue" request="string" response="string">
        <handler name="shipmentQueueShippingRequestHandler" type="AcmeMessageQueueRequestHandlerShippingRequestHandler" method="process"/>
    </topic>
</config>

Here is the ShippingRequestHandler class:

<?php

namespace AcmeMessageQueueRequestHandler;

class ShippingRequestHandler
{
    /**
     * @param string $simpleDataItem
     * @return string
     */
    public function process($simpleDataItem)
    {
        echo __LINE__." $simpleDataItem ";
    }
}

And here is my queue_consumer.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <consumer
        name="shipmentQueue.consumer"
        queue="shipmentQueue"
        connection="amqp"
        handler="AcmeMessageQueueConsumerOrderExport::shippingProcess"/>
</config>

The OrderExport::shippingProcess() is correctly triggered, but the topic handler is never ever triggered, why ?

The main problem is that our rmq is sending JSON object and Magento want a String, so I want to process the queue message from JSON To string.