Not sure what it could be. Maybe one of the other chickens is s carrier? Although of what I can't guess. If all the deaths have been chicks with Hens then I would raise them separate. Integration can be a bit of a bother though. You're sure you don't have a mite problem? They can draw out enough blood to give a mature chicken issues.
I found out on Friday that I have a lice problem. I do regularly check my chickens, but I never suspected an external parasite because they have a clean environment, have access to dust bathing 24/7, and in my four years of owning chickens, I've never had a problem with parasites. But, I realize now that I likely accidentally carried home some lice about a month ago from the wildlife rescue I started volunteering at, since I mainly work with the wild baby birds(they neglected to warn me about carrying home parasites). I had another chicken suddenly drop dead Tuesday, after having eaten and drank perfectly fine a few hours before. I thought this may have been due to my dog, since she killed chickens in the past by running them to death, leaving no marks or anything, but she hasn't done that in over a year and is now comfortable with my chickens. I make the mistake of throwing away the body without extensively searching under the feathers(due to my frustration). On Friday, I found one of my chickens to be suddenly too weak to stand, after having eaten and drank fine a couple hours before. At this point I deeply suspected disease, since literally the only symptom she had was sudden weakness and I had that other chicken drop dead a couple days before. While I was checking over her, I spied many lice running around on her, and a fair amount of eggs of the shafts of her neck feathers. I immediately ran out and got some lice and mite dust, dusted her and cared for her extensively, but she died several hours later. I dusted the rest of my Silkies that night, but waited to dust my other chickens until Sunday, because I didn't find that many lice on them and was trying to find a less messy lice treatment. Unfortunately, a 5 week old chick suddenly got weak too on Sunday before I had the chance to find another treatment, and I lost her. She was in an area that I thought was relatively safe and remote from my most infested chickens. Later that day I noticed that a week old pheasant chick I was keeping inside my home was acting ill, and so I checked him but was unable to find any lice, however I checked the other 5 week old pheasant chick he was housed with, and that chick was infested bad enough to have begun losing his neck feathers from the lice. I didn't notice this sudden feather loss due to how he holds up his neck. I lost the week old pheasant chick today, but I figured that would happen, due to him being so young and this kind of lice being so bad that most of my chickens who died had died before exhibiting any warning symptoms.
Curiously, my adult chickens didn't have a bad infestation. All of my chickens had lice, but none of my adults had eggs on them. The only ones who had eggs/ended up dying were immature chickens or chicks. Now, everybody has been dusted, and most of the chicken areas have had the dust scattered into the ground. I am checking different chickens several times a day and have yet to see any live lice on them, but I'll have to redo the treatment process in a week, due to any eggs that may hatch in that time.
