Every special legendary item like the darkblade helm, the tread on stars (cabal scoutrifle), and others have dropped for me during the heroic strikes ONLY when I used a three of coin. I'm not sure if you have to have a three of coin equipped, but from my experience; the special legendaries dropped when an exotic engram didn't.
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Tbh this is very similar to how it's worked with me got the scout rifles the hand cannon and the warlock helm without grinding just by popping coins and getting no exotic lol