As dawg53 says some chickens handle the heat better than others. I agree Bella doesn't look happy. However, my Luna, a favorelle X, shows signs of heat stress way before anyone else, breathing in gasps through her mouth & holding her wings away from her body. It does look like heat stress to me but you know your chickens way better than any of us could so if you are worried get her checked out.Providing an updated video. A couple of things.
Bella is definitely eating (I did a trial by scratch grain) and I have seen her drinking.
Here she is with Bernadette who has been panting in the heat all day but now it is cooler has stopped panting while Bella continues to gasp.
You can also see and hear Bella honk. She has been doing that for a couple of days now but I put it down to her changing voice as she is growing up. She does it maybe 4-6 times a day. There is no nasal discharge and I don’t hear any breath sounds.