Some sort of distortion when using corner pin

I'm quite new to videography, photography and editing, and wanted to make a short video showcasing my photography. I used corner pin to make a layer of pictures look like it was "on the screen" but when I applied the effect and moved the corners, this distorion came on (Green circle), and left only the bottom right corner un-distorted (red circle). Is this normal or is it a problem with my drivers/hardware? Thanks!