Animated WebP isn't supported (never has been). It's a completely different file format, for some reason, so even the first frame won't display.
We may add support for it in the future, but it's a lot of work to add new animated formats (unless they play via the video APIs), and animated WebP doesn't seem to be taking off, at least on the mainstream web, on top of being a poorly designed file format (hence the first frame not even displaying like it would with a GIF in a program that didn't understand animated GIFs).