I searched for a source but I couldn't find one. My recollection on the topic has to do with how humans perceive luminance within a gradient. Our eyes are better/worse at determining contrast differences between adjacent colours at different luminance. If you search for "luminance gradient" on Google you'll find a lot of scholarly articles on the subject but I was unable to track down one directly applied to graphic design. There is also a semi-famous demonstration [1] of how our eyes perceive gradients as shadows.

In general this isn't an issue since most designers will either just stretch the gradient out or add additional gradient control points to adjust to fall-off manually.

1. http://persci.mit.edu/gallery/checkershadow