I applied the weapon I wanted in World Attunement, but even after visiting the Lost sector multiple times, it does not drop. I ran 50 rounds and only one I wanted appeared. It cannot simply be bad luck, as everything else drops well except for the one I attuned. In fact, this attuning works well for legendary engrams that drop by chance from killing enemies. It seems that only in the Lost sector does this attuning effect either have the exact opposite effect or not work at all.