3 day old cornish cross chicks dying suddenly???

[Cubaniton]

We raise Cornish Cross every year for 4 years, in batches of at least 100 twice a year. We picked up our chicks from the hatchery on 5/17/23, today is 5/20/23 and we have lost close to 15 so far. No idea what's going on and we have never lost this many so early. In fact we normally don't even reach 10% loss over the full course of the batch. We have two large enclosed brooders on our barn and we split the birds into two batches (60 in each brooder) with plenty of room for them to move around or get away from the lamps if they are too warm. Brooder has heat lamps, non-gmo feed, and fresh water. We check on them multiple times per day. The chicks started off all looking healthy and content. We check each chick as it comes out of the box when we take them out and count them into the brooder. All was well on day 1, they arrived, and went into the brooder. Day 2 two or three were found dead, Day 3 two or three more, Day 4 eight were found dead. We have noticed that the ones that don't make it will look lethargic beforehand. Not moving, walking or bothered when we try to move them. None have made it that appear this way. They also appear smaller than the rest (Though, previous to being found lethargic, we have not noticed any looking significantly smaller than the rest).
The ONLY thing that we have done different this year from the last is: normally when they are super tiny we block off part of the brooder since it is so large and as they get bigger we remove the barriers to give them more room. We normally just use a few bags of McFeeder's chips to block off an area. This year we used large plastic swimming pools to contain them. However the pools are large (almost as large as the brooder but slightly smaller) and had plenty of room for the chicks to get away from the heat if they are too warm, so I can't imagine the pool had anything to do with it. We check the heat and adjust the lamps as needed throughout the day.
No other symptoms... no significant bullying, no lesions, no pasty butt, nothing.
Also, both brooders are affected.
Any thoughts?????

Did you requested that the chicks be vaccinated? If so, the ingredients (poisons) in the vaccines could be to blame.