Allen Edward describes some of his procedures to set the diamond knot tight at www.l-36.com. eg. ,but go to http://l-36.com/soft_shackle_9.php for more and other pages there on testing.
I hope commercial makers are taking those or similar steps. Otherwise, the delivered shackle is not a finished product. The only reasons for the tail are to allow further settling and to tighten it further. Nothing I've read suggests, though, that tightening should come from anything but work. An Italian marine fabricator sells these for more than $25, and they have no tails. Using them is easier when the stopper knot is no tails at all.