They are most likely digging into the mulch to try to get to cooler dirt under the surface soil. It does indeed sound like they are heat stressed. they absolutely need shade, breeze and plenty of cool water in the heat of summer. I have read that anything above 100 degrees can be lethal to them. If they are panting with their mouth open, and holding their wings a little away from their body, they are too hot.
I have a large, very airish coop and run a box fan in it all summer. It is cool enough in there that at midday heat they will go in there for the shade and breeze, rather than stay under a bush in their yard. On bad summer days I hose down the coop and the bare dirt just in front of it. In a pinch, I have dunked a bird in a bucket of water, swished her around a bit, and let her air dry. People do things like fill soda bottles with water, freeze them, and leave them lying around where the chickens can cuddle up to them. they also use misters and wading pools for them to be able to stand in water, though mine never would, even though the pool was inside the coop, so well shaded and with a breee to keep the water temp down. I don't actually see how a mister can do much more than cool the soil, since the water runs off them, but some people seem to have good luck with them.
It sounds like you need to figure out a way to set up a fan and create more shade. If your coop is like most built for Midwest winters, it is probably too hot in there for them in the summer day, maybe also at night. I would not force them inside, myself. It's a shame you set your coop up where it is not shaded. Just about anywhere in the US gets hot enough in the summer that they need their coop to be in the shade.