I don't have an answer. I've had a lot of SA fish going back a long time, but not many CAs. I haven't had regular JDs, but I've had EBJDs and I agree, some that seem healthy and active can suddenly get sick for no discernible reason. Don't know whether there's any electric blue genes in yours, regular JDs can have blue spangles or even a dark blue body color, I associate electric blues with a lighter blue or silvery background color, almost like a photo negative of a JD, but it's also true a blue gene carrying EBJD doesn't look electric blue.
As I say, I'm just not an expert on regular JDs, and since I rarely get sick fish, not on an expert on fish diseases (with a couple of exceptions). But since no comments for a couple of days, I thought I'd say what I could. I agree with an earlier post, though-- while it's unusual, I've had the odd fish die for no discernible reason. When a previously healthy fish acts like you've described I assume an internal issue and sometimes try API General Cure and/or Kanamycin. It can be frustrating when there's no discernible cause, nothing seems to work, and all you can do is watch a fish you like go downhill...