I have opened the 2nd secret chest in Encore three times now. Completed the mission on Expert a while ago. Had the necessary quest from Banshee as soon as it was available. Yet the quest status for //CHOIR: UPHEAVAL shows the Expert completion task complete but not the secret chest, and I cannot obtain the additional catalysts (beyond Subsistence Refit) for Choir of One. The Hack the Centaur triumph clearly shows the second chest opened. At the Enclave I can apply Command Frame IV to the weapon. This is on PC/Windows, BTW.