Sorry to hear that, around here my poultry is healthy until it's not, and there's always some underlying problem that I can't see but I know it's there. People will say do a autopsy but I figure why, the bird still wouldn't have made it. Give them good lives, they are way better off than...
My chickens are kinda like yours, not pets but not food, I like giving mine a decent life. When a chicken gets sick around here I watch it, isolate if necessary, than wait about a week to see if they can fight it off, most improve and go about their business, some are the same or worse, the same...