Manzanita (Arctostaphylos manzanita)-Leaf Spots is a fungal disease, which I believe is the problem. There could be as many as 15 different fungi, which have been reported from the Pacific Northwest. For all these foliage diseases; however, airborne or water-splashed spores infect young leaves during wet weather, often in spring.

Symptoms of this disease Gloeosporium arctostaphyli (rust) are spots that at first are purple, then black with purple margins, more distinct from the upper surface. The rust disease alternates between spruce and various species of bearberry including manzanita. The plant eventually turns black and dies. Remove all dead and infected plants to avoid further disease. Plant resistant cultivars such as the 'Massachusetts' strain. Avoid overhead watering or limit it to times when the foliage can dry quickly, which is impossible in the winter. Replant in the Spring when it is dry and amend the soil to help establish the plants before winter. Space plantings to improve air circulation and avoid planting in moist, shady areas.

References:

https://pnwhandbooks.org/plantdisease/host-disease/kinnikinnick-arctostaphylos-uva-ursi-leaf-spot

https://pnwhandbooks.org/plantdisease/host-disease/manzanita-arctostaphylos-manzanita-leaf-spots