Take this as impression rather than expertise. I think the fact that it's new sod would almost certainly eliminate some types of fungus as candidates, and those re the types would be difficult to treat with a fungicide. We had a type of fungus at a place in Denver that appeared as somewhat darker green circles that expended into rings, and the grass in the centre would turn brown. The rings would send up mushrooms sometimes.
A number of fungicides wouldn't touch the stuff. We got a thing that went on a garden hose that poked holes in the sod and injected water below sod level. The combination of the injector and a fungicide cured the fungus.
I remember a warning years ago here in Ontario: 'warning in the Niagara Peninsula. Hairy cinch bug infestations coming from New York State (Guess they didn't mean tourists). From the description, cinch bugs really chew up lawn fast and produce the results you describe. You might try digging up some sod and dirt at the border of a brown spot (if it's bugs, they'd have eaten everything in the centre and wouldn't be there any more). Run the soil through a fine screen and see if you find critters.
I forget what pesticide was recommended for cinch bugs, but I think most of them are pretty wide spectrum agents. For people who don't like to use pesticides, biological agent suppliers sell things like nematodes that control a variety of pests. Of course, skunks do a good job of controlling grubs, but that cure may have a few drawbacks.