It is found that the exposure mode of the ISP parameters of IPC6525 is Auto. Auto (default): The camera automatically sets the shutter speed, aperture value, and gain based on an object's exposure value measured by the metering system.
On the camera web UI, choose Configuration > Channel Configuration > ISP Parameters, and set Exposure mode to Aperture priority. Then, adjust the aperture again. The output images can be darker or brighter.
Aperture priority: Users manually choose a specific aperture value, while the camera then automatically selects a shutter speed and gain to match. This mode is used to shoot static objects.