Hi
I don't know if it just me, but I did max at least 3 Exotic weapon and I never got the achievement (Completely upgrade an Exotic weapon).
I already search for that "bug" and people was saying just log out/log in and it should pop up or if not just wait one week and it will appear.
But It been already near 2 week (maybe more) and I still don't have it.
Can you please help me or rather fix it, because that's annoying that you know you should have an achievement but it doesn't appear.
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In Destiny, go to settings > log out > switch profile > sign in on your profile > choose the character that should've unlocked the achievement > wait a minute. If that doesn't work, you may just have to wait for it to pop. Took almost a month for a couple of mine