On Special:BookSources we embed the parsed version of Wikipedia:Book sources, but this embed is not wrapped in mw-parser-output class.
Because of this the TemplateStyles emitted by the templates used on that page are not working.
Possibly related to T205624: Replace fragile addWikiTextAsInterface("<div class=....>$something</div>") pattern
I think @cscott worked on some of this stuff not too long ago. Maybe he knows how to fix this. I suspect it has to be switched to wrapWikiTextAsInterface, with 'mw-parser-input' or true, but i'm not entirely sure.
There is a local workaround in place, that helps for the float positioning for now, but toclimit is still broken because of this.
I stumbled onto this 5 years later due to colors work, the work around is to add the class ourselves of course onwiki (nothing is particularly harmed), which I've done. I don't think it should be that way so I'm fine leaving the task open.