reporting-engine/report_qweb_pdf_watermark/views/layout_templates.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <!-- Include fix for https://github.com/odoo/odoo/issues/16610 -->
    <template id="assets_pdf" inherit_id="web.report_assets_pdf">
        <link position="after">
            <link href="/report_qweb_pdf_watermark/static/src/css/report_qweb_pdf_watermark.css"
                  rel="stylesheet"/>
        </link>
    </template>
</odoo>