WebP fallback images not loading
All URLs that you put in the srcset attributes have to be valid and have to exist.
Browsers that have support for WebP will try to load the URL from the <source type="image/webp"> and all other browsers will try to load the URL from <source type="image/jpeg">. If the URL selected by the browser does not exist it will not fall back to one of the other <source>s.
You should check the existence of the image files on the server side.