It might be fungus or a disease. I lost a lot of my St. Augustine grass last year in DFW because I watered it too often instead of deeply and less often. This year, I used antifungal treatment and watered once a week, and the grass came back.
Very simple …Release the hounds.
You have chinch bugs. Fungus usually happens in winter or from overwatering in spring and summer. Use a strong insecticide like bifenthrin, cyfluthrin, lambda-cyhalothrin, or permethrin.
Chinch bugs took out my entire St. Aug lawn. Kept treating for brown patch, should have been worried about bug patch
hammer it with water and fert before temps drop too much.
Try raking out some of the dead grass. St. Augustine is quite resilient, and hopefully, the worst of the heat is over. You might also sprinkle some fertilizer in that area to boost growth.