Dawn Chorus will not drop from Legend/Master Lost Sectors if you have not previously acquired it. I have done about 20+ Master Lost Sectors and have not been able to get it to drop. I know that exotic drops are not guaranteed after clearing a Legend/Master Lost Sector however, the reason I know this is a bug a not bad RNG is because I am still getting exotic drops but they are giving me helmets I already have unlocked. In the 20+ runs, I've gotten about 5 exotic drops and they were all exotic helmets I had already unlocked. You can still get the Dawn Chorus to drop from Legend/Master Lost Sectors but this is only possible if you had already unlocked the helmet prior to Season of Plunder.