First of yesterday's cheap faulty items being tested, the Nikon D3400. Metering sensor is not working for the upper part of the photo's frame. I checked each of the autofocus points with the "spot metering" option within the camera looking at an evenly illuminated scene, and established the lower points were all okay.

For the above photo, I switched to "live view" where the camera's main sensor is used to measure the exposure, rather than the metering sensor. A couple of shots required to get correct exposure.

Now have a better knowledge of the fault, and know the rest of the camera is working okay, and is usable, with a bit of a footer.