Hey there…
This is probably similar to the issue where you can still kill bosses after the fact that your character is dead…
If you kill Orobyss in this way (i.e. die before he does), the game currently considers the island as successfully completed and resets the island maps and gives you the stability points as if you did not die. You dont get the chest in Echo of a World - which is correct.
Probably just the logic of success check is out of whack in this edge case scenario.