When manually setting the label net.unraid.docker.icon to a .png everything works as expected and the icon is updated.
When using a webp format image nothing renders at all. When using an svg image the question mark fall-back image is used and when using developer tools I can see the place where the image tag should be is overridden in font-awesome syntax with an <i> tag instead of an <img> tag.
I tried this as a stand alone container, a compose file through the built in unraid compose stack manager, and through portainer with the same results for each. My guess (but I don't know since I don't have access to the server code) is that when svg files are rendered in html they are being loaded into font-awesome tags since many font-awesome icons (or all, not totally sure) are svg format.
My guess with why webp isn't working is that the server isn't set up to handle this format. So it falls back to the question mark.