Tested on versions 0.99.1 and 0.99.0.
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Here's a quick and simple animated WEBP I put together that the XnView has trouble playing.
I also included a comparison video of how it looks like in XnView vs. how it's suppose to look like.

Link to the image.
https://files.catbox.moe/q4sgjl.webp
Video comparison between XnView (incorrect) and Chrome (correct).
https://streamable.com/1zh9q5
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In addition to the poor playblack, notice how XnView displays the wrong resolution for the WEBP file, as well. Perhaps it has something to do with the transparency? No clue.
Also, ignore how they slowly go out of sync. It's not a bug/issue-- I just didn't load the images at the exact same time.