Thank you for this report.
Could you please share more information about this? How the photo was generated, the exact Windows 10 / Photo Viewer build version, how to reproduce, share a file showcasing the issue etc.
Also, consider reporting this problem directly to the Photo Viewer bug tracking platform if there is one. Microsoft has a better chance of investigating and fixing this than the WebP library maintainers.
Photos from Windows 10 Webp Codec Photo Viewer Appears Dark