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Generally, these are cut back in spring but in your area, it should be fine to cut back in fall. Don't take off more than a third from the plant, though each season.

This article will refresh you on the care requirements.
https://www.gardeningknowhow.com/ornamental/flowers/pentas/growing-penta-plants.htm

You seem to be right on the edge of where these can survive in the ground outdoors. With this being said, it is best to treat these as an annual in your area. If they do come back, it will be a plus but is not guaranteed.

These are susceptible to some pests, and this can cause what you are seeing. Can you include photos? This will help me to rule this out. That old saying "A photo is worth a thousand words" could never be more true here.

In the meantime, this article will give you some information on their care: https://www.gardeningknowhow.com/ornamental/flowers/pentas/growing-penta-plants.htm

Pentas plants are frost-sensitive and will not survive winters in USDA zones 8a outdoors, but to save the plants you can overwinter them indoors in containers. Dig up the plants in fall and pot them up in containers. You can also grow pentas plants year-round indoors in bright light.

This looks like a fungal disease. Pull back the mulch and check the soil. Excess moisture and lack of airflow can contribute to this issue.
Remove any dropped leaves from around the plant and discard.
Allow the soil to dry a bit between waterings.

Treat with a fungicide.

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/garden-how-to/info/homemade-fungicide.htm