Smart Books Search doesn't seem to handle collection properly

Graham Criddle

When searching All Books, good results are obtained in Books Search (Smart mode)

But if I specify a collection - in this case a quick collection based on piper - it seems to be just returning results when matching individual words

Is this a current limitation or am I missing something?

Mark Barnes (Logos)

Judging by the highlighting (i.e. the bold text), it looks to me as though the AI hasn't found anything in those collections where it's confident that penal substitution is defined. If it had, it would have bolded the relevant part and lifted it to the top of the results. In this instance, the new search engine works in a similar way to the existing one, except it does a much better job of ignoring irrelevant words and coping with synonyms and multiple forms of the word.

Looking through John Piper's works in the old search, I don't see a clear definition of penal substitution either, even in The Future of Justification.