I have ran through a couple times and even did a couple checkpoints. We managed to get a platinum score but it doesn't check off for the triumph. On my first run and completion i got the Extinction Averted and plat score for each encounter but Atraks defeated didn't unlocked unfortunately.
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Completed Caretaker again with Platinum - 491k score - triumph completed on boss death. So looks like it's awarding Platium medal but the score needs to be higher - I think we were at 145k before the boss died. Hope that helps.