Yesterday I completed every stickerbook of the Xanmeer Genesis set in solstice except for bracers. I killed 6 world bosses while getting random solstice set drops while still not getting the final piece i was missing. I did happen to get the bracers drop in a random writhing wall daily box today, but I wanted to report this anyway in case it's happening to other people and there is some bug with the loot table or other people had the same problem.