See D20650. Currently, Ferret will not return a result if the document doesn't have a title, even if the document matches the search query in other fields (body, etc).

Empty fields are discarded by PhabricatorFerretFulltextEngineExtension and not inserted into the ffield table, but PhabricatorCursorPagedPolicyAwareQuery later does a JOIN against the title field in this table to apply a ranking order.

I think we can fix this by turning the JOIN into a LEFT JOIN and maybe tweaking the ranking logic slightly.