SD-generated images with no metadata?

Hey guys,

I'm conducting primary research on detecting AI-generated images solely by examining their metadata and have locally run SD-1.4 (I have also used Flux.1, but that is online) to generate several dozen photos. However, surprisingly, these images don't contain any meaningful metadata, which I extracted using ExifTools. Essentially it contains file size, resolution, image height and width, as well as specific attributes of the PNG format, totaling 21 attributes.

Why was that? I searched a lot, but nearly nobody mentioned this.

Thanks for any response! This means a lot to me!