This isn't so much a bug as it is (what I would call) an oversight. The Guild Trader's search option doesn't include certain pieces from a set, when they have off-names. The easiest example I can think of is any overland set-piece that came from a quest and (often) have a "unique" name.

So, for example, if I search for "Plague Doctor", [Dres Dirk] won't show up, despite being part of that set. If I'm entering "Plague Doctor" into the search field, obviously I'm looking for items from that set, regardless of their flavor-text names.

This sort of thing should not require an addon to fix, in my opinion.