I updated an old website today using this beta and switched on the option for WebP images. I just noticed on the gallery that the thumbs are WebP, however the enlarged pop ups are Jpg files when dragged to the desktop, which I believe was also reported some time ago by others.
Looking at the project and the asset manager I can see that the thumb and enlargements are all using the same original Jpg files, but only the thumbs are being converted to WebP.