WebP: Colors not preserved in fully-transparent areas

When converting an image to WebP that contains fully-transparent pixels then the color values in the fully-transparent areas are lost. (When converting to PNG the color values are preserved.) Expected behavior is to preserve the pixel values (especially when saving as lossless).

I understand that the rationale for discarding color values in fully-transparent pixels is done to improve the compression ratio. I propose adding a flag to specify the desired behavior. With ImageMagick, it appears that specifying "-define webp:exact=true" will save colors of fully transparent pixels (the default value is false). If this flag is used in GraphicsMagick then I propose changing the behavior such that the default is true when using saving a lossless WebP image.

The attached files show the behavior:
• alpha-test.png → Original image. Smooth gradient with two transparent area (feathered and hard-edge)
• alpha-test-opaque.png → Original image made opaque using GIMP's unerase tool
• alpha-test.webp → Output of "gm convert alpha-test.png -define webp:lossless=true alpha-test.webp"
• alpha-test-opaque.webp → alpha-test.webp made opaque using GIMP's unerase tool