I did the Scourge of the past several times, and I didn't get the anarchy, but every week, someone that already have the weapon drops it again, or someone doing it for the first time get it, but never who done it couple of times
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It's RNG unfortunately. I have more than 70 completions, yet I still don't have the Anarchy while I carried people through that got it the first time doing the Raid.