When an extension introduces a new tag (such as <poem> or <ref>) and the content inside such tag is marked with <section> tags, then when attempting to transclude it from some other page, we get nothing. This doesn't seem to be an issue with native tags (such as <code>) so it probably has something to do with parsing order.
Demo:
- https://www.mediawiki.org/wiki/Extension:Labeled_Section_Transclusion/demobug3
- https://www.mediawiki.org/wiki/Extension:Labeled_Section_Transclusion/demobug4
This is actually a problem for an extension I'm developing that will probably be deployed on Wikiversity, so I'll try to fix it myself, but any feedback and support would be welcome.