My neighbor had a deck built with her new home about 3 years ago. On the areas that are visible, the stringers, there is wood rot in the areas of the screws. The stringers are pressure treated pine, and the deck material is composite material. I'm sure of the brand. It's solid with 2 relief cuts on the back side. She said the screws were specified by the deck manufacturer. The screws are coated with a square drive head. Any ideas what the problem could be?

Thanks