Parser functions [[//mediawiki.org/wiki/Extension:Labeled_Section_Transclusion | `#lsth` and `#lst`]] substitute as documented: they //add //content. But `#lstx`, when it is substituted, //deletes //content on the target page, namely all the content in its section that is under it. So `#lstx` is not working [[//mediawiki.org/wiki/Extension:Labeled_Section_Transclusion#Step_2b:_Transclude_the_page_but_excluding_the_section | as documented]].
For example, try to substitute [[//en.wikipedia.org/wiki/User:Cpiral/sandbox/G |User:Cpiral/sandbox/G]] using lstx: `{{subst:#section-x:User:Cpiral/sandbox/G|d}}`. That should just add the two lines "Linux" and "DevOps" to your test page. It does, but in the process, it also deletes all the content below itself, up to the next wikitext `==section==` marker.