I've run some WebP images from reddit through your site and they all end up looking terrible. If I run them through Squoosh.app (MozJPEG, 75%), I get the expected minimal degradation in quality and a 30%ish reduction in size. If I run the same image through your site (which seems hard-coded to Q90 for quality), I get a minimal reduction in size (if any; to be expected given Q90) and it looks incredibly pixelated.
If I give it a jpeg, it works great: a large reduction in size and I can't even notice a difference (which is kinda what I expect for Q90).
EDIT: If I toggle on the "legacy mode" YCbCr (instead of XYB), the WebP files work great. If I get the jpeg from reddit instead of the webp, XYB is fine.
Try an image like https://www.reddit.com/r/PeterExplainsTheJoke/comments/1o8wo... (you can right click and download the image to get the webp or "download linked file" to get the jpeg). If I put the jpeg into your site with XYB, there's very slight degradation and a 67% savings. If I put the webp in, it looks like it was resized to half or a quarter of the size and then enlarged back up (and it gained 57%). With webp and YCbCr, it gained 63% over the webp, but the image quality is good.
So there's something with the WebP images and XYB that seems like it isn't working.
EDIT 2: I just grabbed an AVIF I have around to test and the XYB seems to have the same problem there as well.