I just looked into our code and found out that we had to change it following our upgrade from Fuji to Helsinki.
What we have done is to put the custom print button of "kb_view_common_header_toolbar" inside the new <j2:if test=...> that is there OOTB and to add the script inside a custom <j2:if test=...> at the end of the script. I've ommited the main part of the code to reduce the length of the code posted, but I could put it as a whole if you need it.
Here is what it looks like:
<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">
<!--A Heisenberg page header as specified in our style guide-->
<g:evaluate var="jvar_is_kb_languages_new_ui" jelly='true'>
var isActive = GlidePluginManager.isActive('com.glideapp.knowledge.i18n2');
isActive;
</g:evaluate>
<j2:if test="$[sysparm_media != 'print']">
<div class="navbar navbar-default snc-article-header-toolbar" role="navigation">
...
<j:if test="${showUpdateAction}">
<button id="editArticle" title="${gs.getMessage('Edit content')}" class="btn btn-default navbar-btn snc-article-header-toolbar-button">${gs.getMessage("Edit")}</button>
</j:if>
<!-- Custom added to add a print Button when there is the required sysparm -->
<button id="printArticle" title="${gs.getMessage('Print Article')}" class="btn navbar-btn btn-primary" style="display:inline-block;" onclick="window.open('/kb_view.do?sys_kb_id=${knowledgeRecord.sys_id}&sysparm_media=print', 'Printer_friendly_format', 'resizable=yes,scrollbars=yes,status=yes,toolbar=no,menubar=yes,location=no')">${gs.getMessage("Print")}</button>
</div>
</j:if>
</div>
...
</j2:if>
<!-- Automatically opens the print option of the browser when the sysparm get passed using the previous button -->
<j2:if test="$[sysparm_media == 'print']">
<script>
window.print();
</script>
</j2:if>
</j:jelly>
