I've just had a similar experience: one treated hive appeared to be doing well, plenty of stores and bees and brood. Then many bees started falling to the floor around the hive, a few days later a small swarm left and took up residence on a neighbour's roof before I could collect them, and two weeks later the original hive appears to be dead. I'm going to go in and check in a few days when I'm off work.
The colony did appear to have CBPV in May when they arrived, as well as a varroa bomb which caused a short brood break, but they appeared to have made a recovery and were split in late July for swarm control and easily filled a 12x14 with stores and brood. Fortunately the smaller split is still doing well. I'm guessing the failing colony tried to supercede the old queen and failed. I'll go in and check for life in a few days when I'm off work...maybe I can rescue some comb or maybe they will surprise me with their resilience again.
Just like the OP asked, any advice about how to treat/use the stores or comb would be appreciated.