I died as the final boss was killed during the harbinger mission. For some reason the game decided to set my respawn location as before the parkour section leading to the final boss encounter.
I was not able to make it back to the boss room in time and could not collect the dropped rewards. Nothing has appeared in my postmaster and it looks like the rewards are gone for good.
I don't know if this is intended or not figured I should post it as an issue just in case.
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Seems like it is intended to pick up the chest for it to give you loot (but the respawn location could be changed so that you can get it even if you die).