It's almost assuredly a Destiny bug (Destiny)

by JDQuackers , McMurray, PA, Thursday, April 09, 2015, 16:22 (3307 days ago) @ Xenos

This would most likely be related to how Bungie has implemented achievements for Destiny on Xbox One. For whatever reason, they decided not to integrate with the Microsoft APIs for achievement tracking. I suspect that this is because Bungie wanted TOTAL server-side control over the conditions of the achievements (why? Who knows...).

But certain achievements that would normally be tracked locally on your Xbox (and provide real-time feedback in the achievements app) don't get marked as complete until the whole shebang is complete and the Bungie servers have "blessed" the achievement unlock. This combined with the fact that the Xbox One achievements are ALSO activated remotely means that we are probably seeing a huge bottleneck in the achievement pipeline between Bungie and MS