If you have all the titan armor why would you open it on your Titan? Fated engtams have always been class specific. You're mistaken about having received a Warlock piece with your Titan.
As has been stated many times before, exotic engrams only award weapons or class-specific armor for whatever class buys/earns the engram. If your titan has all exotic armor & all non-quest exotic armor, then you shouldn’t be buying the engram on the titan as you’ll only get repeats. You may have bought one of the exotic armor that he sells for another class, but it did NOT come from exotic/fated engram.