I know this is a very old topic, but wanted to post my answer in case it solves someone else's problem. I was having this same issue and the solution was to allow the xmlpackage to be run by the engine in the package declaration.
<package version="2.1" displayname="Package Name" debug="false" includeentityhelper="true" allowengine="true">
You can double check if the package is actually allowed to run by the engine by accessing http://www.yoursite.com/x-packagenamewithoutxmlconfig.aspx. Please note that the url is for the package name only without the .xml.config extension. If the package is not allowed you will see an error message, otherwise the package content should show up.
After that the package should show in the topic without problems, I'm actually showing it in an entity page's description
I'm in version 9.5, not sure about backward compatibility.