Red twig dogwoods are susceptible to several fungal leaf spot diseases that can produce these symptoms on the foliage -- fortunately not fatal to the plants. This tends to be more common in moist, humid conditions and can vary from year to year based on weather conditions. It looks like your plants are situated close to a wall, which could be creating a microenvironment with limit air circulation/more humidity. High heat reflection from the wall might also cause stress on the plants.

Fungicides will not cure symptoms that are already present. You can remove the most heavily damaged leaves or allow them to shed on their own, rake them up, and put them in the trash (not compost). This will help to remove fungal inoculum from the area. Avoid overhead watering to keep the foliage as dry as possible. You might also consider thinning the plants out or moving to an area with better air circulation.

https://extension.umd.edu/resource/fungal-leaf-spots-shrubs

Christa