According to this article: Using Cloudflare Polish to compress images – Cloudflare Help Center
Polish can only go from PNG/JPG to WebP, not the other way around:
To ensure WebP is not served from cache to a browser lacking WebP support, disable WebP at your origin web server when using Polish.
However, when host a WebP image, turn on Polish, and view that image using Internet Explorer 11, CF transparently vends a JPG to the browser.
This is great! But, it seems to conflict with the documentation. Any clarification here?