Live Data fails to render in PDF export on 17.10.x

I traced it down to this line in meta.vm (from 17.10.12):

$services.rendering.render($uix.execute(), 'xhtml/1.0')

This code that renders the org.xwiki.platform.html.head UI extensions, has been moved in recent versions to {htmlheader.vm}}

The mentioned line produces the <script type="importmap"> in view mode and an empty string when the PDF export print preview is loaded. The culprit seems to be the outputSyntax=plain from the PDF export print preview URL. This seems to force a plain text rendering, instead of the requested 'xhtml/1.0'. This doesn't seem to be the case on master. The output syntax specified on the render() call should have priority over the output syntax specified on the HTTP request.