Sorry about your chicks, it’s always a kick in the gut to go from watching the antics of fluffy little happy guys, to pulling the deceased out of the brooder.
This may or may not apply here, but I’ll throw it out in case it helps someone... most of us know, the fumes from non stick cookware can kill birds indoors in enough concentration. Many of the floodlight and heat lamp style bulbs have a “anti-shatter” coating on them now, which if used in close proximity can kill birds, especially young stressed chicks in an environment meant to be free of drafts. I now use only brooder plate style heating. I buy off season so they aren’t so spendy.
Another thing I’ve found is if chicks have been started on medicated diet, it’s better to not switch out right away, and I’ve also learned to spend the money in “organic” chick starter for anything I’ve hatched at home because it’s more expensive, but I have zero issues with pasty butt. Could be any number of reasons, but on the scale I hatch, it simplifies my life to just pay a couple bucks more and not hassle with it.
I hope you get to the bottom of it.