Featured images on posts marked as "unreferenced unused"

I tried to remove images from the “Unreferenced Unused Images” list, but it ended up removing the Featured Images on several of my posts. I get the impression it did this for images where the original images already had the exact same standard width for featured images. So the posts refer to the original featured image instead of a resized version. ANy thoughts on why this happens, and why Image Cleanup refers to the images as “unused” when they’re clearly set as featured images for posts?

https://www.ads-software.com/plugins/image-cleanup/