I've run Master mode Vox Obscura and acquired and completed the catalyst, but the Tank Master triumph is still not available to claim. The attached Imgur album has my records for the completed catalyst and the incomplete Tank Master triumph.
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That happens if you leave before the mission is completed. Opening chest is not the mission completion. The mission completes after the dialogue.
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Sounds like you (or team lead) left the mission after opening the chest but before the mission ended (which is after all propaganda dialogue ends).