I’d relax. It sounds like your chicks are well acclimated.
I raise my chicks in a brooder in the coop from Day 1. One end of the brooder is kept warm but the other end cools off as it will. I’ve seen that far end frozen at times. Yeah, it can really cool off. When it’s that cold the chicks stay pretty close to the heat. But when it is just cool, they play in that far end a lot, just going back to the heat when they need to warm up. They get used to cooler temperatures and feather out faster.
A few year ago, I took chicks from that brooder when they 5 weeks old and put them in an unheated grow-out coop. The overnight lows were consistently in the mid 40’s. About three nights later the overnight low hit the mid 20’s. The chicks were fine. That grow-out coop had good draft protection on the floor where they were.
Your chicks are older than mine were and it sounds like they are well acclimated. As long as they have decent draft protection they should be fine.