Although it is possible to create headers with multiple lines, using the XML Editor in backend siteaccess, creating content as such using the API is not possible.

Steps to reproduce:

• Create some content, using the admin siteaccess. In a XML Block field, select heading 2, type "multiple line", press shift+enter, "header"
• Publish the content
• Verify that the content both in backend as in front end shows a single h2 tag with 2 paragraphs within.

• Attempt to create the same content using the API

  $contentCreateStruct->setField( "name", $name );
  $xmlText = <<< EOX
  <?xml version='1.0' encoding='utf-8'?>
  <section>
  <header level="1">Multiple line
  header</header>
  </section>
  EOX;
  $contentCreateStruct->setField( "description", $xmlText );
  (...)
  

=> Line break is stripped. Any attempts to include one fail, since header is setup only to be able to contain "custom, strong, emphasize, link, anchor"