Most likely because you’re using a crop sensor (APS-C) lens on a full frame camera, which causes vignetting or a black border around your image.
The black border effect isn’t an issue with your lens adapter, it’s because the size of the lens is designed for a smaller camera sensor. So when you use the APS-C lens with a full frame camera sensor, there is a border around the outside where light from the lens cannot reach. This appears as a black band in the image.
Here is an analogy to help you think of this in a different way: Imagine your APS-C lens is a 6×8 photograph and you want to frame it. Your full sensor camera is like a 10×12 picture frame. If you want to use this frame for that photo, there’s going to be some blank space around it.
To avoid this happening, you can use crop sensor lenses with crop sensor cameras, and use full frame sensor lenses with crop sensor cameras although the image will be cropped by the size of your camera sensor.