We had three pullets pass separately, between ages 16 weeks to 10 months that were fine one day, then croaked suddenly. We follow organic living, clean chalet coop, everything is to the "rule" of raiding good hens. The each were healthy but died suddenly a few months apart. All three lost ability to stand. One had gleet
The other two who knows. They could have had a parasite.
I wouldn't be concerned unless you have others due suddenly within the week. In meantime, clean coop and hen boxes, provide Organic ACV (with mother) in their waters - 1 TBSP per gal, if providing mealworms or larvae limit to only once or twice a week as treat. Too much protein causes pasty butt. Scratch is good in winter for warmth and encourages foraging in free roam area, you don't need a lot. I provide herbs every other day... I also provide some left over raw veggies like cabbage, celery, carrot, lettuce, broccoli. As treat only. For pullets -layer pellet crumble, for hens reg pellets are their main feed. We provide two waterers for 14 hens year round.
That's the best you can do.