The problem was that the innerwiki template included the tiddler $:/plugins/tiddlywiki/railroad, which was omitted from the wiki. Unexpectedly, missing tiddlers were rendered by the jsontiddler widget as an empty object {}. The fix is to always include the title when the tiddler is missing.
Also cleaned up the template to remove unneeded tiddlers
From the readme:
This plugin enables TiddlyWiki to embed a modified copy of itself (an "innerwiki"). The primary motivation is to be able to produce screenshot illustrations that are automatically up-to-date with the appearance of TiddlyWiki as it changes over time, or to produce the same screenshot in different languages
jeremy@jermolene.com