Webp can be really annoying once you hit certain encoding edge cases.
One customer of mine (fashion) has over 700k images in their DAM, and about 0.5% cannot be converted to webp at all using libwebp. They can without problem be converted to jpeg, png, and avif.
We have never been able to resolve it better than knowing this:
Certain pixel colour combinations in the source image appear to trip the algorithm to such a degree that the encoder will only produce a black image.
We know this because we have been able to encode the images by (in pure frustration) manually brute forcing moving a black square across the source image on different locations and then trying to encode again. Suddenly it will work.
Images are pretty much always exported from Adobe, often smaller than 3000x3000 pixels. Images from the same camera, same size, same photo session, same export batch will work and then suddenly one out of a few hundred may become black, and only the webp one not other formats, the rest of the photos will work for all formats.
A more mathematically inclined colleague tried to have a look at the implementation once, but was unable to figure it out because they could apparently not find a good written spec on how the encoder is supposed to work.
Webp has a maximum pixel dimension size of 16383 x 16383.[0]
[0] https://developers.google.com/speed/webp/faq#what_is_the_max...
Just out of curiosity, what's the problem libwebp has with them? I wasn't aware of cases where any image format would just cross its arms and refuse point blank like that.