Do we need to kill each boss only once to get its loot permanently in the vault during the season ?
For expemple ; If i killed Terros Heroic last week but this week i have killed all heroic boss except Terros, do my 3 heroic slots have a chance to contain Terros Heroic loot at the end of this week ?
The loot can come from any boss regardless of which one you kill, so even if you fight tooth and nail to down Raszageth and you fail for weeks, you can still get loot from there any reset, provided you killed at least 2 bosses.
Worked like this in M+ for me, so I have no doubt the loot is the same random in raids.