Activit - update email task field values

I try to update fields of a mail service task by expressions, and strangely the "subject" field is not updated when the others are.

Extract of my xml BPMN

<extensionElements>
        <activiti:field name="to">
          <activiti:expression><![CDATA[${emailTo}]]></activiti:expression>
        </activiti:field>
        <activiti:field name="from">
          <activiti:expression><![CDATA[${emailFrom}]]></activiti:expression>
        </activiti:field>
        <activiti:field name="subject">
          <activiti:expression><![CDATA[${emailSubject}]]></activiti:expression>
        </activiti:field>
        <activiti:field name="charset">
          <activiti:string><![CDATA[UTF-8]]></activiti:string>
        </activiti:field>
        <activiti:field name="html">
          <activiti:string><![CDATA[<p>Hellol etc…</p>]]></activiti:string>
        </activiti:field>
      </extensionElements>
    </serviceTask>

My update from Java (de is my DelegateExecution instance) :

de.setVariable("emailTo", "someone@mycompany.com") ;
de.setVariable("emailSubject","This is the request number : " + bean.getId() );

When the mail is received, the "to" is updated but the "subject" of the mail is empty. How can I test the sending of the mail from within Activiti process ?

Thank you if you have an idea.