Our plugin is active on over 100K websites hosted on thousands of different webhosting providers. I understand that it can be frustrating for you not to work as you expected “out of the box” but in most of the cases contacting support can help you figure out the solution to your problem very quickly.
Also, I understand that you believe that your hosting provider “makes it really easy to flush the server side cache” but maybe next time when you encounter an issue with an image that won’t change just give it a try and add “?anything” at the end of the image URL. This way you’ll force the webserver to read the actual image on disk and ignore the cache.
I’m telling you this also because you wrote “The image quality was really bad and unusable on all three settings” which, in my opinion, proves it that you’re having a caching issue. The Lossless option will return pixel-identical images as you can check yourself easily here.
Have a nice weekend! ??
Alex