Hey there, I’m not much of a transmog fiend however I AM an owner of many quest related achievements spanning near 2 decades. Quest rewards not unlocking all of the options’ mog falls incredibly flat to the greater population from my own experiencing and from what I can gather from others.
To my understanding I believe at Blizzcon it was advertised alongside the other new features, and was a consistent, and possibly reported, thing that existed during alpha/beta.
As someone who’s done Loremaster and near all zone quest achievements to majority completion, I cannot imagine having to do that up to 8x over depending on what the collector wants. I STRONGLY suggest looking in to this again, I believe it would be a great good will gesture and make many collectors happy. Thank you 
