I have done hero, legends, and a master nightfall with no drops of PRE ASTYANAX IV (this week's bow) and any other time I would get the drop way before id have to run a master, usually a couple of hero's and I'll get it but even a master didn't drop it and now I'm curious that there's something going on. could be just really bad rng but at least I said something if it isn't.
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update: got the drop after 7 hero 2 legend 1 master. seems like just bad RNG but it shouldn't take that long to get Imo. you should get them dropped in grandmasters at least once for collections. and it's not just about getting it for collections but it's easier to farm for a roll you want of a normal version with vanguard engrams than getting unlucky drops of grandmasters.