Previously…what you’re describing is EXACTLY as intended for previous expansions. If you run Siege of Orgrimmar or Hellfire Citadel, three and two expansions behind Azeroth, Legacy loot is enabled and you’ll receive the full cache from the boss. Legion and BfA on the other hand will only reward you what you’d loot as in a 10-man raid, whether or not you actually are a body among nine other people.
Well, now incoming problem solved for Legion.
https://us.forums.blizzard.com/en/wow/t/legacy-loot-coming-to-legion-instances/478878