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I believe that to be normal behavior. The reason being is aperture priority is set so that once the aperture is setup, the shutter speed automatically adjusts so as to capture what the camera believes to be proper exposure. When auto ISO is introduced, it automatically adjusts/raises ISO so as to provide, at the least, the minimum shutter speed required to achieve what the camera believes to be proper exposure. As you have set the maximum ISO to 6400, the camera will go no higher, but with the aperture you have set, when the camera is unable to provide proper exposure at your minimum shutter speed, it will have no choice but to compensate by falling below your set minimum shutter speed so as to maintain proper exposure.