Heat will exacerbate issues in an animal that has a compromised/damaged respiratory system. If the animal is currently sick, it will become that much more pronounced. In an animal that previously suffered lung damage, but appears to be fine ~ it will present with difficulty breathing and lethargy when the heat and humidity climb. I had a calf that had pneumonia, the vet told me that in the heat of a Mn summer, this calf should only be out from dusk til dawn, and should be given banamine to ease its breathing (this was after the pneumonia was gone, but the lungs were scarred ~ anyone buying that calf from me would not have known there was a previous issue until the heat came, then they would wonder. Maybe that is what you bought? I don't know.).
Heat also makes a bad situation worse by, as we all know, causing dehydration. Maybe the calves get lethargic in the heat, and are slow to get to water? I have no experience with calves and the type of heat you are dealing with so I can't help you more. I don't know anymore about bloat than I have read on here, and I am sure you have read too.
Maybe the banamine. Maybe electrolytes in the water.