Incubating eggs, air cell vanished?

I’ve been incubating eggs from my flock, I started with 12 and down to six now. I had 5 early deaths, looked to be between day 6-9. I candled everything when I placed the eggs into lockdown yesterday, and I found this one egg where the air cell was completely gone. The chick was clearly not moving. First 2 pics show that egg and 3rd shows one that looks normal. I opened it up, and the chick seemed properly developed. TW: dead chick on last 2 images. I’m wondering what might have gone wrong to cause this? I’m also not sure why I’m having so many early deaths either. I’m keeping my temp at 99F and the humidity between 45-55%. I do find that the humidity in this incubator fluctuates a good deal, especially with the air temp outside the incubator. We keep our house around 74F during the day and 72F at night. The humidity in the incubator drops a degree or 2 overnight usually and goes back up during the day. I’m also going off what the incubator reads on its own. I don’t have a seperate thermometer/hygrometer in there.