WebP isn't served on AMP websites
AMP doesn't support
<picture> tags, so that's the reason why ShortPixel Image Optimizer isn't able to serve WebP images on AMP websites if you have activated the option Using the <PICTURE> tag syntax.
There is a workaround available published by @westonrouter on Github but is considered anti-pattern, so we wouldn't recommend this approach.
- Go to https://gist.github.com/westonruter/332abdb2adefc6b204ad6fcc1beecedf
- Click on Download ZIP.
- Upload it to your WordPress dashboard, on Plugins > Add New > Upload Plugin.
- Clear your cache: How to clear WordPress cache
We're looking into other possible solutions to this issue and we'll implement them if we find any.