You should be okay at f13, It could be the lens sample, if it is possible, rent or borrow a similar brand of 35mm lens, put the camera on a tripod, with a remote release connected to rhe camera, and shoot a same scene using both lenses at the same settings, as well as trying an entirely different lens to use as a means to eliminate the brand as having sample issues.
Sometimes lenses are soft, sometimes the sensor in the camera is a slight bit tilted which throws things off, sometimes weird stuff happens.
You should be mostly be okay until around f16 - f22 before noticing artifacts in pixel peeping, depending on the lens. Of course if you are photographing window screens or things like that, you can get issues at f1.2.
