I don't get any exotic arms in lost sector. I do the lost sector in less then 10 minutes and no get nothing.
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Heyo! The rewards at the end of the legendary and master lost sectors are RNG-based, meeting that you are not guaranteed to get a reward at the end of the run. It takes everyone a different number of runs. I, for example, got mine after about five runs, but have not gotten a piece on my hunter after about 30 runs. It's purely RNG-based. Keep at it and good luck!