Does anybody in your team have Rally? It is known to break stealth when leaving and reentering the AoE.
They don't use it while I set traps. I go in stealth and set traps and my party which lately had been just alistair is waiting back out of sight. I set the trap in stealth and at some point the darkspawn end up surrounding me. I have full stealth and full traps. They don't attack me. But something triggers them to run right to me and surround me. Then I am stuck and have to toss bombs. The trap may be doing little bits of damage while they stand there like zombies on top of me and I can't move, but the whole things is screwy. It happened several times. I cannot set a trap without them either completely avoiding it or surrounding me but in a way that seems that they don't see me but are drawn to that area.
I can do stealth for backstabs very easily. I can even at times go stealth in front of them as a disengage and confuse tactic that maybe the ones right near me will stay attacking but others lose sight of me. I can toss bombs in stealth. But traps don't seem to work for stealth.