Wikimedia Bugzilla is closed!

Sorry for opening an empty bug, I unintentionally posted the form while typing.

The issue is that {{FULLPAGENAME}} returns the same value as {{PAGENAME}} in article namespace (empty prefix): the colon is missing. It would be needed in template transclusion of articles, for example {{ {{FULLPAGENAME}}/test }}. In other namespaces it works correctly, in articles it tries to find a template of that name; you will have to write {{ :{{[FULL]PAGENAME}}/test }}.

Or am I wrong and this behavior is intended?

{{FULLPAGENAME}} is also used for display purposes.

You can just do {{:{{FULLPAGENAME}}/test}}

{{:Template:Foo/test}} will still do the right thing.