I noticed on the app it tells u how far through both raids and the weekly heroic strike u are but there's no indication that u have completed the nightfall on any of your characters. Yes it might seem dumb that u can't remember if u have done it or not, but still doesn't seem to much of an add to the app to make it that more helpful.
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And it doesn't seem like it's shows which parts of the raid you have completed rather it just shows how many parts you have completed.