I'm in the same boat as far as reds. It's too sad when you have to cull your brooder baby due to loss in quality of life. I've had cocci in a year old hen before, but that came with severe anemia. (She was extremely pale)This is sadly true. One of the reasons I will no longer get sex links. They are bred to lay more than they naturally should and it destroys their bodies. So sad. I have several going through it right now. However I have also noticed that my older sex links are WAY more prone to cocci at this age, than ANY of my other adult chickens I’ve ever had. They get lethargic, lose their appetite, and have runny poop. I would personally say that it may very well be that. Corridor is what is needed for that. I would also get her on a oregano supplement, in her water after corrid treatment, and get her to eat wet cat food as soon as she will take it, and mix in some turmeric. It will help with inflammation from any reproductive issues likely at her age. And the oregano will help to fight infections like salmonella which they are also prone too.
Cocci treatment definitely doesnt hurt. Fecal float and an oral drench too, though not all dewormers are created equal. It's just one of those things with reds... All of mine fell ill the same winter - one would finish the molt and be ready to get back on the lay and that's when it'd strike them... One after the other. One had ascites and notable lumps, poor thing.