Sounds fishy to me, not sure what would kill them after a week though. I'm sorry you suffered this one time let alone two!
Switch to a Mama Heating Pad setup. The whole "95°F for the first week, 90°F for a week, etc" is bull shit. Have you ever seen a hen heat their surroundings to 95°F?? We naively followed that rule with our first 12 chicks in 2012 in a bathtub. I had the heat lamp on a remote reading reptile thermostat. That room was UNBEARABLE! They were moved out to the barn horse stall coop at just under 4 weeks.
What they NEED is a heat source they can go to when they get cold. A hen is best because she moves around with them. MHP is second best. I'm on batch #6 of chicks now and the last 5 have started with MHP in a big cardboard box for the first day and night. If I had a broody hen they were moved out to the coop the next night. One more night inside if no broody and then moved out to the coop with the MHP. All of the chicks saw temps down in the 50's and 60's at night, they just hung under the hen or MHP when they got cold. They never see anything close to 90°F ambient their first month.
I have them on paper towels on newspaper when in the cardboard box, regular shavings in the brooder area of the coop, same shavings as the rest of the floor.
These chicks are 4 days old in the brooder area of the coop, temp out in the barn that day according to a Google search was high 70°F, low 60°F. Notice that they aren't even under the hen
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10 days old, movable heat source High that day 75°F, overnight low 45°F.
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I too recommend a mama heating pad (MHP) if you don't have a broody Hen. Works very well. I've brooded week old chicks outside with nighttime temps in the 20s with just a MHP. Of course they were in a protected brooder/coop, but they were running around like a bunch of nuts in there LOL
A GOOD Broody Hen is, imho, the BEST way to brood chicks. Amazing to watch and actually easier (for me) LOL I don't have to fiddle with setting up the MHP, don't have to use electricity - just provide "fuel" (fresh feed/water) for Mama and chicks