So I thought I was going to have this problem too, because Brave Collector says I have 4 more weapon quests to complete, Arcite was only offering 3, and it said I already had 10 out of 12 weapons under the individual weapons in the general brave triumph. So apparently you can mark those individual weapon triumphs as complete before you complete the Arcite tasks if one of the weapons drops. I was seeing "Succession" as complete in my triumphs but I knew I didn't do the quest, Arcite wasn't offering it, and I couldn't attune to it. After googling for quite a while, I randomly decided to look through all my quests. It was buried in page 2 or 3 of my quests. Into the Light released a day or two before I left for a 3 week work trip, and I must have claimed it then forgot about it. It got buried in my quests when I returned home. So all that to say, check through your quests and see if you have any of the weapon quests somewhere. Also see which weapon(s) you can't attune to and that should give you a hint what to look for. If you can attune to all weapons then I'm not sure.

Same here, the exact same issue.. I keep looking to see if I missed something :/