After testing on live it's really behaving like Argo said : Same Name is Same Face !
This clearly looks like a client side behavior using name as "Seed" for Face Model...
I wandered around Tutorial where there are most recent mobs and where Live Devs could need to use some alternate behavior, but it's clearly hard coded, for most "Realm Invader" they tried to change equipment and made them wear helmets so we don't notice they are all clones
Which means even Live don't have some work around to change Face Model with Same Name, this behavior can be clearly noticed on Neutral NPC's in fact, like Guards or Sentinel, they all look alike when Same Race and Same Gender, if you want to randomize mobs with No Name or Same Name you'll have to use different Model (Race), or Different Gender or different Equipment...