Simple API and associated alpha

I'm using the simple API to read an 8-bit RGBA PNG.

Here is the image:
https://github.com/kurocha/images/blob/master/test/Images/fixtures/bacon.png

I'm writing some tests to check the behaviour in comparison to WebP.

Here is the documentation I'm reading:

When an alpha channel is present it is expected to denote pixel coverage
of the color or luminance channels and is returned as an associated alpha
channel: the color/gray channels are scaled (pre-multiplied) by the alpha
value.

I've written a test to load this image, and where the alpha channel is
0, I also expect color channel to be zero.

Here is the code that uses the simple API to read the pixel data:
https://github.com/kurocha/images/blob/2cb3397cd5ed6966cf1fd852dca855909b80c15e/source/Images/PNGImage.cpp#L43-L50

Here is the test:

https://github.com/kurocha/images/blob/2cb3397cd5ed6966cf1fd852dca855909b80c15e/test/Images/Test.Transparency.cpp#L30-L38

However, in many cases this is not true. Is this a bug? Am I
misinterpreting the documentation?

Thanks
Samuel

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