Topic on Help talk:Subpages

I have a template Template:Example that contains

{{/subpage}}

and a corresponding Template:Example/subpage. When Template:Example gets transcluded into a document, however, it appears to try to transclude Document in question/subpage (which doesn't exist) rather than Template:Example/subpage. Where is this behavior documented? I can't find anything in any of the links on this page, or on any google results.

I ran into the same problem, it's as if Topic:Uuen4em4k7ji6q01 is used internally when expanding the subpage path.