Why header value isn't inserted into Apache Camel's `to` url parameter of the route in Spring DSL?

We have a route that will accept the kafka.KEY and use that as mqtt url parameter to send the data to the right topic.

<routes
    xmlns="http://camel.apache.org/schema/spring">
    <route id="KafkaToMQTT">
        <from uri="kafka://mqtt?brokers=localhost:9092"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=start"/>
        <log message="Headers ${header.kafka.KEY}"/>
        <to uri="mqtt:mqtt?host=tcp://localhost:1883&amp;publishTopicName=try${header.kafka.KEY}"/>
        <to uri="log://camel.proxy?groupInterval=3&amp;level=INFO"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=stop"/>      
    </route>
</routes>

In the log messages I see the ${header.kafka.KEY} correctly, while in the mqtt I'm getting the topic as literally try${header.kafka.KEY}

What is the reason for that, how to make the header to be used there?

To avoid that the right element instead of to should be used, that is toD.

toD concatenates the url correctly, so the right route XML is:

<routes
    xmlns="http://camel.apache.org/schema/spring">
    <route id="KafkaToMQTT">
        <from uri="kafka://mqtt?brokers=localhost:9092"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=start"/>
        <log message="Headers ${header.kafka.KEY}"/>
        <toD uri="mqtt:mqtt?host=tcp://localhost:1883&amp;publishTopicName=ESP_02/try${header.kafka.KEY}"/>
        <to uri="log://camel.proxy?groupInterval=3&amp;level=INFO"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=stop"/>      
    </route>
</routes>