There’s an issue with the Dalaran Defender’s Plate Armor set from the pre-patch event not showing up as a “set” when you visit a transmog NPC. It shows up as a set when you do Shift+P and go to the appearances tab though. When I got the last piece of gear I needed to complete the set (the gloves) and equipped it, I did not get the standard “this item has been added to your collection” and “you’ve completed this set” notifications.
This is an issue that seems to be isolated to the plate set, as the cloth, leather, and mail sets display properly on transmog NPCs.