That's incredibly odd... Though it does reaffirm my opinion that we aren't needing synths. It sounds like you've encountered an extremely rare bug. I'd either try and contact Bungie in some way to trouble shoot, OR - Test the method I've had to do to fix my achievement unlocks and see if it cures it. Level 20, all subclasses, the prestige achievement... None popped on completion. So what I did was go to settings, "switch account", sign into a different profile, pop THEM into the game, then "switch account" using their profile to go directly back to mine. All achievements then popped at once. Not sure if that would cure whatever you're experiencing, but it's worth a try?