Ref T13091. In Differential, if you provide a query and "Sort by: Relevance", we build a query like this:
((SELECT revision.* FROM ... ORDER BY rank) UNION ALL (SELECT revision.* FROM ... ORDER BY rank)) ORDER BY rank
The internal "ORDER BY rank" is technically redundant (probably?), but doesn't hurt anything, and makes construction easier.
The problem is that the outer "ORDER BY rank" at the end, which attempts to order the results of the two parts of the UNION, can't actually order them, since rank wasn't selected.
(The column isn't actually "rank", which is selected -- it's the document modified/created subcolumns, which are not.)
To fix this, actually select the fulltext columns into the result set.