My best guess would be some form of switch grass, Panicum virgatum. There are several red or or purple bladed varieties but I would hesitate to assign a specific cultivar name to yours as self seeded grasses rarely come true to form.

Actually, I'm now thinking it's a sign of phosphorous deficiency. If it were a characteristic of the plant, you'd see the purpling on all leaves (as in, a switchgrass purple bladed variety). You don't here. Just a few leaves. That's a hallmark of phosphorous deficiency. We live in an area with alkaline soil, and this soil has never been amended with anything, so that makes such a deficiency more likely.

My very first thought was a physiological issue. Not a species with red markings. I don't know a thing about American grasses. But I know I see this phenomenon on grasses here where we have no native striped species. Usually it's caused by a fungus.