Hard to say without more info as to species and conditions. Hot weather can cause the flower stalks to turn brown while the roots themselves are perfectly fine.

Most western penstemon languising in eastern wetter gardens, start tanking about now...with heat, wet, and humidity.

Hopefully you have a species adapted to the east coast.

I bought the plants at a local native sale at a park and they're supposed to be right for the area. Should I cut them back or just let them be like all my other natives that have seen their day for their season? I haven't cut any back nor thinned them out.

Cutting back-or not-will have minimal impact on the outcome for these plants. Do you have Penstemon digitalis? This white-flowered form is the one in bloom around here right now. I don't know that I've ever seen it brown up following bloom time, but I'm sure enough heat and drought could do that. Yet, I doubt you're experiencing either weather condition at this time. Hmmm?

BTW, the beardstongue, or Penstemon digitalis, considered native around here is quite likely actually native to areas to our south, having moved north with the clearing of land, farming, etc. IOWs, a native which is probably not native! Nice plants though.

+oM