ShortPixel Adaptive Images doesn't serve WebP images, why is that?
First of all make sure you have this option active in Settings/ShortPixel AI:
If you do have this setting active then WebP images should be served. Please note that the URLs of the images won't change so URL will still have the original (e.g. .jpg) extension but the actual image will be WebP. The reason for this is that changing the image extension in URL would have required an additional request and this is not optimal.
Take for example: https://cdn.shortpixel.ai/spai/w_604+q_lossless+ret_img+to_webp/https://blog.shortpixel.com/wp-content/uploads/2019/04/Boost-Page-Speed-1024x685.jpg
If you look at the image details you'll see that it is actually a WebP image as you can see below:
If the URLs seem correct, and you have the previous setting enabled, and you still don't see the WebP files being served, try enabling the option Settings > ShortPixel AI > Lazy-load the backgrounds. WebP requires lazy-loading, so there's a chance some background images need to have this setting on to be able to be served in WebP format.