Glad to see them outside - my meat birds go outside at a week old. They have shelter outside available to them, and can enter a coop. I do not think it was a heat issue. Because they are growing so fast, they produce a huge amount of heat.
Don't know if this will help, or not. But when I feed mine, I wet down the the feed, I just add water, going for 'good sand castle" consistency. I think that dry feed is very dehydrating in the amounts that they eat. I feed in the morning, and mid afternoon, they do not have feed available 24/7. And I turn out the lights, and they sleep through the night. They are starving in the morning, so do be early down there.
These are not long lived birds, but rather a particular strain that grows fast. Report it where you got them, I would more strongly suspect a genetic failure than either feed or disease. There are several strains out there, see if you can get a different one next time.
Mrs K