I dont have a stealther but Iv been in Ops, we are standing outside the bosses agro range waiting (always waiting in Ops) and one of our scoundrels comes up BEHIND us stealthed and the boss kills him, its totally bizarre, iv not seen it for ages coz we learn but its a really old "bug"
That is intended, not a bug. Wouldn't be much of a challange if someone could sneak past all the bosses.
If you have never had a stealth character you don't realize how much of a "cheat" it really can be sometimes.
I get away with murder in some 2 or 4 man heorics that are at level content because of it.