I've also looked on TTC's search interface and they don't even have a preset for it. i.e. you start typing the name in the search box "Sword-Singer's B" and it doesn't even have a drop-down selection. Typing "Sword-Singer's Bow" it acts like it doesn't even exist. I've tried manually searching under the "Sword-Singer's" search and absolutely NO bows whatsoever. Not garbage Training, no named bows, nothing. I dropped 380k ap into the Alik'r regional vendor and got bows for the other 2 sets, but no bows for S-S.
I have to assume this is a bug. While it does seem the game's designed to detect the type of build you are running and give the complete opposite garbage sets constantly, along with monitoring trends and avoiding giving the popular set-pieces (weapons you use especially) and traits especially (noticed more training junk from pledge boxes since Transmute was added....), NEVER dropping a bow from a less-popular set is ridiculous. I just wanted to try the set out to supplement a pve set I always have on. Since my other main 5-piece is dungeon set, getting it's bow will be harder (I've been through 50+ times and still nothing) and if S-S doesn't hold up, the entire endeavor was a waste. And farming Automaton isn't something that's very appealing at the moment.